4.1 Rattlesnakes versus squirrels and deathJearl Walkerwww.flyingcircusofphysics.comJune 2009 When a California ground squirrel encounters, say, a gopher snake, the squirrel waves its tail to scare the snake. However, when it encounters a rattlesnake, it waves its tail but also increases the temperature of the tail to scare the snake. This type of squirrel recognizes a fundamental difference between the two types of snakes …. a rattlesnake can not only see the squirrel with its eyes but can also detect the squirrel via the infrared radiation (IR) emitted by the squirrel.

Infrared radiation and visible light are types of electromagnetic waves but in a different range of wavelengths. Everything around you emits infrared radiation due to its surface temperature. Because of this link with temperature, IR is often called thermal radiation.

You can sense the IR emitted by, say, a hot stove because your skin warms as you absorb the energy of the IR. But you cannot see in the infrared range of wavelengths because your eyes cannot form images in that range as it does with visible light.

However, a rattlesnake can form images with IR. On each side of its head, between its nose and an eye, it has a pit with an IR sensitive membrane lining the back of the pit. The image is not as refined as the images produced by the snake’s eyes, but in the first year of its life, a rattlesnake learns how to correlate the crude images in the IR with the finer images in the visible. The IR detector is so sensitive that the snake can sense a change in the temperature of its surroundings by as little as 0.001 degree. Thus, it can easily sense a mouse that runs near it.

When I used to cave explore in West Texas, I worried a lot about the IR sensitivity of rattlesnakes, which often crawl into a cave entrance to escape from either a Texas summer or a Texas winter. In the dim light of my helmet lamp, I could not see much as I crawled into an entrance, but a waiting rattlesnake could easily pick me out by the IR emitted by my body.

When a California ground squirrel encounters a rattlesnake, it heats its tail to trigger that IR detection in the snake. By making the tail more apparent, the squirrel seems larger (and thus more dangerous) to the snake. As Professor Stefan Pulver of Brandeis University put it, the squirrel sends a message to the snake, “Back off. I’m big and hot.”

Here is a video shot in IR that shows a squirrel’s encounter with a gopher snake. The tail is relatively dark because the squirrel knows that heating up the tail will do no good because gopher snakes do not have IR detectors.

Here is a video that shows the squirrel’s encounter with a rattlesnake. This time, the tail is lit up in the IR due to its temperature increase, because the squirrel knows that the rattlesnake has IR detectors.

The researchers who made these videos (see the paper by Rundus, Owings, Joshi, Chinn, and Giannini that is listed below) also built a robotic squirrel to test a rattlesnake. The snake displayed less desire to approach the robot when the tail was both waved and warmed than when the tail was only waved.

As I describe in The Flying Circus of Physics book, a rattlesnake’s IR detection can still trigger a strike for as long as an hour after the snake has been killed, even if the snake has been decapitated. Although this may sound like an urban myth, it has been documented in both medical and news reports. For example, recently a man was bitten by a rattlesnake after his son had cut off the head with a shovel. “When I reached down to pick up the head, it raised around and did a back flip almost, and bit my finger,”

4.3 Death by thermo-balling and asphyxia-ballingJearl Walker www.flyingcircusofphysics.comOctober 2007 In The Flying Circus of Physics book I describe research concerning how Japanese bees protect their hive from an invading giant hornet, which preys on the bees. Once alerted, hundreds of the bees will form a ball around the hornet and then increase their activity so as to increase their body temperature from the normal 35ºC to 47ºC or 48ºC. The bees can tolerate the increased temperature but not the hornet, which dies as result of this thermo-balling as it is called. One of the following URLs link you to a very interesting video from The National Geographic Society about thermo-balling, but skip it if you love giant hornets.All objects radiate infrared radiation because their surface molecules oscillate. All objects also absorb infrared radiation from their surroundings because the molecules in those surroundings oscillate. If an object is warmer than the surroundings, it loses more energy than it absorbs and thus to maintain its temperature, the object must generate energy.Such is the case with ball of bees. In normal circumstances, they are warmer than their surroundings and must generate energy (from the transformation of food) to maintain their body temperature. In the ball, they must generate even more energy because of their temperature increase. The advantage of balling is that the surroundings of any of the interior bee are other bees, and so an interior bee absorbs as much infrared energy as it emits. However, the bees on the ball’s surface lose more infrared energy than they absorb. To increase the temperature of the whole ball requires that all the bees generate enough energy to offset the radiation loss on the surface. Typically, a ball of bees can kill a hornet in about 20 minutes. In the sixth edition of the textbook I write, I calculate that death in 20 minutes requires an average increase in energy of about 1 joule by each bee, which is reasonable.A recent research paper describes a similar balling defense of Cyprian honeybees against a hornet, except that death is not directly attributed to the increase in temperature within the ball by the bees. Instead, the bees somehow know how to interfere with the hornet’s breathing and thereby suffocate the hornet. The temperature increase itself does not roast the hornet to death because the Cyprian hornets have a fairly high tolerance of higher temperatures, matching the tolerance of the bees themselves. Still, the higher temperature within a ball of bees may contribute to the death by decreasing the carbon dioxide release by the hornet.http://www.youtube.com/watch?v=hpcHH1EpTZM videohttp://www.youtube.com/watch?v=EuAfbt8-7VE&feature=related videohttp://www.boingboing.net/images/beefu.jpg photo · Ono, M., I. Okada, M. Sasaki, “Heat production by balling in the Japanese honeybee, Apis cerana japonica as a defensive behavior against the hornet, Vespa simillima xanthoptera,” Experientia, 43, No. 9, 1031-1032 (15 September 1987)· Akre, R. D., and D. F. Mayer, “Bees and vespine wasps,” Bee World, 75, No. 1, 29-37 (1994)· Ono, M., T. Igarashi, E. Ohno, and M. Sasaki, “Unusual thermal defense by a honeybee against mass attack by hornets,” Nature, 377, 334-336 (1995) · Koeniger, N., G. Koeniger, M. Gries, S. Tingek, and A. Kelitu, “Observations on colony defense of Apis nuluensis Tingek, Koeniger and Koeniger, 1996 and predatory behavior of the hornet Vespa multimaculata Perez, 1910,” Apidologie, 27, No. 5, 341-352 (1996) · Anderson, C., G. Theraulaz, and J.-L. Deneubourg, “Self-assemblages in insect societies,” Insectes Sociaux, 49, 99-100 (2002) · · Halliday, D., R. Resnick, and J. Walker, Fundamentals of Physics, John Wiley & Sons, 6th edition, page 446 (2003) · Milius, S., “Balls of fire: bees carefully cook invaders to death,” Science News, 168, No. 13, 197 (24 September 2005) · · Ken, T., H. R. Hepburn, S. E. Radloff, Y. Yusheng, L. Yiqiu, Z. Danyin, and P. Neumann, “Heat-balling wasps by honeybees,” Naturwissenschaften, 92, 492-495 (2005)

Want more references? Use the link at the top of this page.

4.4 Huddling Jearl Walkerwww.flyingcircusofphysics.comFeb 2009 Why do armadillos (perhaps a dozen of them) huddle at night? Why do emperor penguins (perhaps thousands of them) huddle during the Antarctic winter? Armadillos, emperor penguins, and many other warm-blooded animals huddle during cold weather in order to stay warm. If, say, an emperor penguin stands alone, it can lose a significant amount of thermal energy by conduction (to the ground), convection (to the air, especially if the air is moving), and thermal radiation (to the cold environment, including the sky). In the harsh environment of the Antarctic, where temperatures can be -40°C and wind speeds 300 kilometers per hour, individual penguins could perish due to the energy loss.

The huddling is most important when the penguins breed during the winter. The resulting egg is incubated almost exclusively by the father, who keeps the egg from freezing by balancing it on his feet for months in the continuous winter darkness, with a pouch-like layer folded down over the top of the egg. During this incubation time, the father must fast because his food is in the water. So, with no incoming energy from food, he must be part of a huddle or his thermal energy losses will drive him into abandoning the egg to search for food in order to stay alive. He needs that protection even though he lowers his metabolic rate to conserve energy.

By huddling (up to 10 penguins per square meter), the penguins significantly decrease the average loss of thermal energy due to convection and radiation—only the penguins on the perimeter still suffer large losses, but they still benefit from their nearest neighbors.

Here is another way of saying all this: If you place many individual “warm cylinders” in a cold environment, the total thermal energy loss can be very large because the total surface area through which the energy is lost is large. However, if you bundle those cylinders to make one very wide cylinder, the total surface area is less, and thus the energy lost through the surface is less. For example, if 1000 penguins tightly huddle, the rate at which they lose energy by radiating it to the environment is about 15% of the rate if they stand apart.

The other great advantage in huddling is reduction in the wind chill factor because the penguins inside the huddle are exposed to the wind only along the top of the huddle. Of course, the penguins on the perimeter are much more exposed, especially those on the windward side. So there is a constant flow of penguins from the windward side, with penguins shuffling around the perimeter and into the huddle. Thus, the penguins gradually take turns in being on the perimeter and in being in the worst position for wind chill.

Of course the penguins know nothing about textbook physics and could never work out a single homework problem, but nevertheless they have an intuitive grasp of the thermal physics needed to survive in that terribly harsh environment. There is nothing like the possibility of death to sharpen your wits about physics.

4.4 Survival strategies of emperor penguinsJearl Walkerwww.flyingcircusofphysics.comMarch 2015 In the harsh winter environment of the Antarctic, where temperatures can be -40°C and wind speeds 300 kilometers per hour, individual penguins will perish due to the energy loss by conduction (to the ground), convection (to the air, especially if the air is moving), and thermal radiation (to the cold environment, including the sky). To survive, they huddle in groups of thousands.

Forming such a huddle is most important when the penguins breed during the winter. The resulting egg is incubated almost exclusively by the father, who keeps the egg from freezing by balancing it on his feet for months. During this incubation time, the father must fast because his food is in the water. So, with no incoming energy from food, he must be part of a huddle or his thermal energy losses will drive him into abandoning the egg to search for food. Once a chick is born, it is kept warm inside a low pouch just above the father’s feet.

By huddling (up to 10 penguins per square meter), the penguins significantly decrease the average thermal energy loss due to convection and radiation—only the penguins on the perimeter still suffer large losses, but they still benefit from their nearest neighbors. Here is another way of saying all this: If you place many individual “warm cylinders” in a cold environment, the total thermal energy loss can be very large because the total surface area through which the energy is lost is large. However, if you bundle those cylinders to make one very wide cylinder, the total surface area is less, and thus the energy lost through the surface is less.

Studies have revealed that waves frequently travel through a huddle, allowing huddles to move gradually downwind. Initially the waves were thought to originate with movement of penguins on the perimeter. Those penguins will indeed move around a huddle and into it because they are the ones most exposed to the cold and wind. The interior penguins were less likely to initiate a wave because they were kept much warmer by their neighbors. However, recent studies show that the waves can originate with any penguin that happens to take a step forward, whether on the perimeter or in the interior. The situation is likened to a traffic jam in which one car moves a short length, and that allows a series of adjustments by other cars. The purpose of the waves is likely to fix any “defect” in the huddle structure (slightly open space where the thermal energy can be lost). They are also used to merge two or more huddles to increase the thermal stability.

4.5 Space walking without a spacesuitJearl Walkerwww.flyingcircusofphysics.comJanuary 2014 Some researchers speculate that a person could briefly walk in space without a spacesuit (as an astronaut does in the movie 2001: A Space Odyssey) without dying. If the walk is far from the Sun, would the astronaut feel cold? Is there more danger to the astronaut than simply a lack of oxygen? Here is the answer in The Flying Circus of Physics book:

One reason why the temperature of a room feels comfortable is that the infrared radiation sent to you from the walls and the infrared radiation sent to the walls from you are approximately equal. So, you gain energy at about the same rate as you lose energy. If the radiation to you drops significantly, you feel cool or cold. If you were to walk in deep space, away from your spacecraft, there are no walls and so you would feel very cold very fast. The rate at which you would lose thermal energy is about 800 watts. However, the lack of oxygen would be of much greater concern.

Your exposure to the vacuum would also be of concern. When water is exposed to a vacuum, it first boils (some of it vaporizes) and then it freezes. You have a lot of water in your body, and . . . well, maybe we should think about something more pleasant. Here is a demonstration of what would happen, using marshmallows instead of a person:

"At NASA's Manned Spacecraft Center (now renamed Johnson Space Center) we had a test subject accidentally exposed to a near vacuum (less than 1 psi) in an incident involving a leaking space suit in a vacuum chamber back in '65. He remained conscious for about 14 seconds, which is about the time it takes for O2 deprived blood to go from the lungs to the brain. The suit probably did not reach a hard vacuum, and we began repressurizing the chamber within 15 seconds. The subject regained consciousness at around 15,000 feet equivalent altitude. The subject later reported that he could feel and hear the air leaking out, and his last conscious memory was of the water on his tongue beginning to boil."

x4.6 Fingers in molten leadJearl Walker www.flyingcircusofphysics.comNovember 2006 First, a serious caution. This stunt is really, really dangerous, and don't you dare think about trying it. I have been badly hurt by it, but because I was the one who first did it in modern times, my name is associated with it and so I still do it. Back in the 1970s, when I toured the U.S. and Canada with my Flying Circus of Physics talk, I showed the stunt to lots of physics instructors. They were amused by it, but most of them had the good sense (sense of self preservation) not to repeat it. However, a few failed the good-sense test and went off to hurt themselves in front of students. As described in The Flying Circus of Physics, the stunt requires that I dip my fingers first into water and then into the molten lead. Lead melts at 328 degrees Celsius but I take care to get the lead even hotter so that there is less chance that the lead will solidify when my fingers enter it. Its temperature exceeds the so-called Leidenfrost point for water (about 200 degrees Celsius), a temperature named for Johann Gottlieb Leidenfrost. He did not dip his fingers into molten lead (thus, he was smarter than I am, but about 300 years ago he wrote about how a drop of water can last much longer on a very hot metal spoon than on a moderately hot spoon. His explanation was that when the metal is very hot, the bottom of the drop vaporizes and then the vapor layer holds the remaining liquid water above the hot metal surface, thereby slowing the transfer of thermal energy to the water. So, the water can take a lot longer before it vaporizes. Do you that many people can remember where they were when something really dramatic happens? Well, I can remember exactly where I was standing when I discovered a copy of the article by Leidenfrost, in the engineering library at the University of Maryland. (All right, all right, I fully realize just how nerdy this makes me sound.) When I was a graduate student at Maryland, I always worked on my Ph.D. dissertation up until midnight and then switched to working on the original version of The Flying Circus of Physics, until I passed out due to sleep depravation. In those days before search engines, looking for FCP ideas was demanding because I had to physically search page by page through books and research journals. Curiously, some of the old physics books of about 1900 described how a carnival daredevil would stick fingers into molten metal, but I shrugged off the accounts, thinking the daredevil somehow faked the stunt. After all, how could a person touch a very hot surface without the finger being burnt? When I discovered Leidenfrost's article, I immediately realized how the stunt was done. If the finger is wet, the water immediately vaporizes when it enters the lead and then the finger is coated with a thin vapor layer that slows the transfer of thermal energy to the finger. I say "slows" and not "stops" because thermal energy will still be transferred to the skin, so the vapor layer's brief protection is indeed brief. After I graduated and came to Cleveland State University as a professor, I decided to try the stunt myself. As my common sense screamed at my foolishness, I first touched some molten lead with a wet finger and then plunged four wet fingers into it. The stunt proved to be a great treat for my classes. (Ah, let's watch the professor burn off his fingers! He won't be able to write exams after that!) My second "Amateur Scientist" article in Scientific American magazine was about the Leidenfrost effect and the fingers-into-lead stunt. While making measurements and drafting the article late one night at school, I wondered: Is the water on my fingers really necessary for a plunge into the molten lead? I was so tired that I threw aside all rational thought and stuck a dry finger down into the molten lead. What is the shortest amount of time that you know about? A microsecond? A nanosecond? A zeptosecond? Well, whatever your answer is, I realized in less time than that just how incredibly stupid I was. The pain rushed up my finger, faster through the fingernail than the fleshy part of the finger, presumably because either the fingernail conducts better or it is simply faster to heat. "Yes," I screamed to no one but myself (because, thankfully, no one else was at school that late), "water is indeed necessary." If you want to see numbers and graphs about the Leidenfrost effect, here is a link to an essay I wrote for the textbook Fundamentals of Physics (David Halliday, Robert Resnick, Jearl Walker):http://bcs.wiley.com/he-bcs/Books?action=mininav&bcsId=3673&itemId=0471758019&assetId=120126&resourceId=11027&newwindow=true

The photograph of me in the essay is old and was taken under pretense because a few weeks later it appeared, not in a Swiss newspaper as I had been promised, but in a tabloid newspaper here in the U.S. However, the photographer was so good that even after I realized that I had been tricked (when someone told me that my photograph was in the National Enquirer), I still wanted the photograph. In the blog site (click on the blog choice in the menu at the left here), you can find a link to an excellent (and recent photograph of me as my fingers enter molten lead, splashing the lead toward me (and my brand new Flying Circus of Physics tee shirt, which ended up with bits of lead melted into the fabric). Again, do not dare try this stunt yourself. There are several subtle dangers---you can end up badly burned or even blinded. In the coming months, I shall tell you more about those dangers and how I painfully discovered them. If you would like to see an old video in which I perform the stunt, go to https://www.facebook.com/video.php?v=10150099476500074&set=vb.339329532602&type=3&theaterThe last stunt I did on that television show was the molten-lead trick. By the way, the stunt has been included in Ripley's Believe It or Not:

4.6 Leidenfrost videosJearl Walker www.flyingcircusofphysics.comMay 2007 Dean Baird, who teaches physics at Rio Americano High School in Sacramento, California, has a wonderful video involving the Leidenfrost effect (see the URL listed below). The most common example of the effect is when you throw water drops onto a hot skillet. If the skillet is only moderately hot, the drops evaporate within seconds. But if the skillet is very hot, a drop may last well over a minute because the vapor released from the bottom of the drop supports (and thus protects) the remaining drop, slowing the rate at which thermal energy is transferred to the drop.

Baird’s demonstration is similar. His video show a hot brass ball that he has suddenly immersed in water. A Leidenfrost vapor layer surrounds the ball, delaying the transfer of thermal energy from the ball. (Such delay is a common concern to engineers in industries that depend on the rapid cooling of hot metal surfaces by water.) Occasionally, when the vapor layer collapses and liquid water touches the ball, there is a sudden vaporization. The resulting expansion of the water as it flashes to vapor sends a sound wave through the container and out into the air. Play Baird’s videos with the volume control on your computer turned up. When I did that, I jumped with every sudden vaporization. Way cool!

In March 2007, when I gave the Flying Circus of Physics talk at UCLA, someone in the audience recorded (probably with a cell phone) the last minute of the demonstration where I dip my fingers into molten lead. The video is posted on YouTube. Search for “Jearl Walker” or just visit the URL listed below. Hey, I am just happy that I did not lose my fingers. Watching that on YouTube would have really depressed me. Well, I guess losing my fingers would have been pretty depressing too.

4.6 Liquid nitrogen stunt and serious physical damageJearl Walker www.flyingcircusofphysics.comApril 2007 Liquid nitrogen evaporates at about -200ºC. However, if liquid nitrogen is spilled onto, say, a floor, the drops will not evaporate immediately in spite of the floor’s much higher temperature. The delay is due to the Leidenfrost effect: As a falling drop nears and then touches the floor, the underside evaporates, producing a layer of nitrogen vapor that supports the remaining liquid drop. Tens of seconds may be required before sufficient thermal energy passes through the vapor layer from the floor to evaporate the liquid. Until then, the drops can slide frictionlessly over the floor, supported by their vapor layers. In 1994, S. Chandra and S. D. Aziz of the University of Toronto reported on experiments in which they produced such vapor-supported nitrogen drops on copper and glass layers. On the average the drops lasted longer on the glass, supposedly because they initially cooled the glass more and thus slowed the energy transfer across the vapor layer. They also reported something I saw when I wrote about the floating nitrogen drops in my column in Scientific American of August 1977: When the surface is rough with imperfections that are as tall as the vapor layer’s thickness (0.35 micron to 50 microns), the imperfections produce bubbles in a drop that significantly reduced the drop’s lifetime. In the Flying Circus book and elsewhere, I describe a stunt that I used to do. The stunt is very foolish and resulted in the microscopic fracture in the enamel of my top two front teeth. (You cannot see the fractures but my dentist was horrified when she put her strong inspection light on the teeth.) I could have been hurt much worse, as someone recently was. In the stunt, I would put a small amount of liquid nitrogen in my mouth and then breathed out over the top of the drop. The nitrogen floated on a Leidenfrost layer of vapor and thus did not freeze my tongue, and the expelled breath created a wonderful plume. I felt like a medieval dragon, something I had always longed for as a youth because dragons are were so cool. However, the last time I performed the stunt in my physics class, the liquid nitrogen made contact with my top two front teeth for too long, and the enamel fractured into a “roadmap” (as my dentist put it) during thermal contraction. I was always very, very careful not to swallow any of the liquid. However, after the nitrogen-in-the-mouth stunt was performed recently by someone else, a spectator apparently drank the remaining liquid nitrogen while the performer’s back was turned. The liquid and then the resulting cold vapor made prolonged contact with the spectator’s stomach and esophagus and he required immediate emergency surgery and suffered severe permanent damage. So, do you see my message here? Even if you see a description of the stunt, don’t do it. Maybe you could live with fractured teeth but, if you accidentally swallow the liquid nitrogen, could you live with only a fraction left of your stomach and esophagus?If you would like to see an old video in which I perform the stunt, go to www.myspace.com/flyingcircusofphysicsand click on videos and then choose episode 5. The video was shot in the early 1980s, as you can tell from my long hair. The video is also posted in Facebookhttps://www.facebook.com/media/set/?set=vb.339329532602&type=2Go to the fifth to last video (which lasts 19:27 minutes).

4.11 Freezing salty water and microwaving frozen foodsJearl Walker www.flyingcircusofphysics.comOctober 2006 If you freeze a container of salty water, the salt acts as an antifreeze, delaying the freezing and decreasing the freezing point (the temperature at which freezing occurs). Wherever the ice forms, the salt is forced into the regions that are still liquid. This is one reason why frozen foods can be thawed in a microwave oven. The microwave radiation (an invisible form of light) can heat food if the food has liquid water. The heating process involves the forced oscillations of water molecules in the water---the motion can break bonds that hold the molecules in temporary groups. As groups reform, the energy that went into breaking bonds is then transferred to thermal motion of the molecules; that is, the water becomes warmer. When frozen food is put into a microwave oven, the microwave radiation can warm the food because there are pockets of liquid water throughout the food in spite of the low temperature. Food that has been in a common freezer may be at a temperature of -10 degrees Celsius, which is below the freezing point of pure water, but because of the salt content (or other ingredients), the food still has pockets of liquid water that can be heated by the microwave radiation. The heating should be done slowly and at a low oven setting. Otherwise, the liquid water pockets overheat (and overcook the surrounding food) while the rest of food is still frozen. Slower heating at a lower setting allows the thermal energy from the liquid pockets to spread into the frozen sections, thawing those sections so that they too can be warmed by the microwave radiation.

4.12 Hot water freezing before cooler water ─ the debate continuesJearl Walkerwww.flyingcircusofphysics.comJuly 2009 If equal amounts of water in identical open containers begin at different temperatures, one very hot and the other cooler, which will form ice first when they are placed in the same subfreezing environment? Surprisingly, in some circumstances the initially hot water freezes first.

As I explain in The Flying Circus of Physics book, the result was known to Aristotle and to people in cold climates. However, scientists largely scoffed at the validity of the result until the 1960s, when a Tanzanian student, E. B. Mpemba, asked his high school teacher why an ice-cream mixture will freeze more rapidly in a freezer if it is inserted when hot rather than after it cools to room temperature. The teacher believed the claim only after Mpemba demonstrated the effect with water, and the result is now known as the Mpemba effect.

Since the effect was named and published, many researchers have offered explanations for the counterintuitive freezing behavior and a number of experiments have been conducted. However, no single explanation has been convincing, and reproducing the effect in a controlled lab environment has been challenging. My own amateur attempts at showing the effect is the subject of this month’s “Article of the Month.”

However, a recent publication by J. I. Katz of Washington University in St. Louis, Missouri, offers what I think is the first credible explanation, and Katz lays down the challenge for it to be tested. He points out that tap word contains various solutes that can lower the freezing point of water. Many of those solutes become less soluble if the water is first heated, and he especially points out that solubility of calcium bicarbonate (which is commonly the reason why tap water is “hard”) decreases if the water is heated.

So, suppose we start with equal amounts of water in two containers at room temperature. We heat one container to, say, 60 ºC and then we place both containers in a freezer in order to freeze the two water samples. Intuitively we would argue that the cooler container would freeze first because the initially hot container obviously starts out farther from the freezing point. So, it would require additional time for its temperature to fall to the starting temperature of the other container. Intuitively then, the never-heated water would freeze before the heated water.

Katz counters this intuitive argument by focusing on the solutes. The heating removes the solutes in one container, and thus that water freezes at 0ºC. However, the never-heated water still contains its solutes and thus its freezing temperature is below 0ºC. This effect is most important in the liquid water near the freezing line (where the ice formation progresses through the liquid water). As water freezes, it expels solutes and thus their concentration is highest just in front of the freezing line. There the freezing point of the never-heated water might be as low as -10ºC.

Moreover, the rate at which thermal energy is lost to the freezer at the freezing point will differ for the two containers. Suppose the freezer is set at -15 ºC. One the heated water begins to freeze at 0 ºC, there is a 15 C º difference between its temperature and the freezer’s temperature. That difference determines the rate at which the thermal energy is transferred to the freezer. When the never-heated water reaches its freezing point, the temperature difference might be only 5 C º and thus the transfer rate will be slower. Katz argues the freezing point suppression and resulting slower energy transfer that this is the real reason why the never-heated water might freeze before the heated water.

You might want to check his argument. For example, start with the equal amounts of water at room temperature and then heat one of them. Then allow it to cool back to room temperature. If you put both in a freezer, how long do they take until ice begins to form? How long do they take to fully freeze? Keep in mind that you get meaningful results only if you run the experiment many times.

4.16 Throwing hot water into very cold airJearl Walker www.flyingcircusofphysics.comFeb 2008 If you place a pot of boiling water outside in subfreezing weather, tens of minutes are required before the water begins to form ice and tens of minutes more are required to complete the freezing. However, if you throw the water from the pot into the air, most of the water will freeze before it reaches the ground. Below I provide a number of links to videos that show people tossing very hot water (usually at the boiling point) into the air. (Nothing on television. No money for a movie or a rental video. So, what can we do tonight? Oh, I know. We’ll throw hot water into cold air. Call up our friends. They’ll want to see this. No, wait. We’ll just put it on YouTube.)

Why is the cooling and freezing rate so much faster with the thrown water than with the pot of water? The difference is due to the rate at which thermal energy can be transferred from the interior of the water to the surface. In a pot, the transfer is relatively slow because it depends primarily on convection within the water, that is, the flow of warmer (lighter) water upward and cooler (denser) water downward. Thermal energy is also transferred (but rather slowly) via conduction, as the thermal motion of the molecules is transferred from the molecules in the water interior to the molecules on the top surface or along the pot’s walls. The rate of energy loss by either process will depend on the surface area of the water on the top and along the walls.

When the water is thrown into the air, the volume of water does not change but the total surface area increases, and thus so does the rate of energy loss. Also, there is now only a short distance from the interior (now the interior of a drop) to the surface (the surface of the drop). So, the water can lose energy so quickly that the drops freeze in mid-flight.

Water must expand when it freezes (and that is why an ice cube floats in liquid water). So, when the liquid drops suddenly freeze, their abrupt expansion can produce a pulse in the air. The combination of those pulses can be a tinkling sound, as reported by inhabitants or explorers in very cold regions. In the videos linked here, I can hear sound but I do not think it is this tinkling, which is surely too soft to be picked up by a microphone on a video camera positioned several meters from the water drops. Rather, I think the sound is due to the impact of the frozen drops on the ground or snow, much like the sound hail makes when it lands.

If you repeat this demonstration, let me know if you can hear tinkling before the water reaches the ground.

4.17 Freezing of water on iciclesJearl Walker www.flyingcircusofphysics.comJanuary 2007 Although they have been studied for a very long time, icicles are still not well understood. Here is one curious feature: An active (still wet, still growing) icicle has a thin central core of liquid water that extends down to the pendant drop at the tip. The top of that core gradually freezes, which means that it loses thermal energy so that the water molecules can become locked up in solid ice. But to lose thermal energy, the water must send it toward a point with a lower temperature. So, it cannot send the energy downward through the liquid water because that water is at the freezing point or slightly warmer. It cannot send the energy out horizontally, because the liquid water sheathing the icicle is at the freezing point. The only direction left is upward: The core gradually freezes by sending thermal energy up to the icicle root at the top and only if the temperature of the root is below the freezing point.

Recently, researchers at the University of Arizona have investigated the carrot-like shape of an icicle, noting that common stalactites have the same shape. In particular, the investigators developed a mathematical model for icicle growth. A thin sheath of water coats the outside of an icicle, draining toward the tip. As the water descends, some of it freezes to the ice surface, adding to the icicle width. Again, freezing can occur only if the water can lose some of its thermal energy. The energy cannot go into the icicle because there is no lower temperature in that direction. It must come horizontally outward to the external surface of the water layer and then be transferred to the air. On a windy day, the transfer is easy to picture. How about on a calm day? As air molecules take up some of the energy, the density in that air decreases slightly and then that air begins to rise because of buoyancy. Thus, convection of the air up along the side of the icicle removes the thermal energy. The rate at which the energy is removed sets the rate at which the icicle can grow in size and therefore the ultimate shape of the icicle.

Want more references? Use the link at the top of this page.x4.17 Ripple migration on iciclesJearl Walker www.flyingcircusofphysics.comFeb 2011 Although icicles have been studied for a very long time, several puzzles remain, including their geometric shape and the curious ripples that develop along the sides of most icicles. Recently Antony Szu-Han Chen and Stephen W. Morris of University of Toronto posted a paper about their studies of icicle growth under controlled situations. They employed an enclosed refrigerated box within which the temperature was maintained within a fraction of a degree in experiments where the temperature was set in range from -7ºC to -21ºC. The icicles grew from water that was slightly above the freezing point and which was slowly pumped onto a sharpened wooden support that pointed downward. The water slowly drained down the support, forming an icicle. As more water drained along the sides and froze, the icicle grew in width and length.

The support slowly rotated (one rotation per 4 minutes) so that the researchers could observe and video record all sides of an icicle through a window in the container. In addition they could adjust the flow of air past an icicle. The air motion should presumably be important because the freezing process along the side depends on the transfer of thermal energy from the draining water to the air immediately next to the water. The rate of that transfer is increased if the air flows along the icicle, carrying away the thermal energy.

An icicle grows as water runs down its surface, adding new thin layers of ice on the sides. At the tip, the remaining liquid forms a succession of pendant drops, each detaching and falling when it becomes too large to cling to the tip. Each drop leaves fresh ice at the tip, causing the tip to grow downward.

Previous mathematical modeling predicated that if left undisturbed so that the water flow is uniform over the surface, an icicle should form the idealized shape that you would draw for an icicle. You could argue that the ideal shape is more likely to occur in still air because moving air might disrupt the symmetry of growth around the sides of an icicle.

Chen and Morris found that icicles more closely resembled the ideal shape when the air was gently stirred. When the air was still, an icicle was likely to develop multiple tips that can resemble long fags, as you can see in my photograph here of an icicle hanging outside my living room.

The ripples on icicles have fascinated me since I was a child, and my interest was renewed a few years ago when K. Ueno of Nagoya University reported that the ripples gradually move upward along the icicle. Here is a photo of big and rippled icicles hanging outside my living room.The ripples are presumably begun by some chance nonuniformities on the surface. Then, for reasons I do not understand, they develop into a pattern with a spacing (a wavelength) of about 1 centimeter. Once set up, they probably migrate upward because of how the draining water tends to cling as it passes a ripple. On the top side of the ripple, the water moves more slowly and more of it has a chance to freeze. On the bottom side the water drains more quickly and less of it has a chance to freeze. In the meantime the icicle as a whole is growing outward. Thus, the ripples appear to move up the outward growing icicle.

Chen and Morris confirmed this upward migration but also found that generally ripples did not form on the icicles with the more ideal shapes. We might guess that if conditions promote that ideal shape, they also tend to disallow the slight nonuniformities needed to initiate the ripples.

Icicles remain somewhat mysterious, so if you live a climate where they appear, why not measure and monitor some icicles over their growth and eventual death. Instead, you could set up a refrigerated chamber to run controlled experiments as Chen and Morris did.

x4.18 Snow sheet curling off roofJearl Walkerwww.flyingcircusofphysics.comMay 2012In this uncommon formation, we see a sheet of snow that is gradually sliding off a roof. Initially, a layer of snow had accumulated on the roof. The snow crystals along the bottom must have then been partially melted by heat from the roof (from the room or attic beneath the roof). The resulting thin layer of liquid acted as a lubricant, allowing the snow layer to slide downward. Usually the entire snow layer comes sliding off the roof, but here there must have been enough friction to allow only a gradual descent. Also, snow throughout the layer has been partially melted (by sunlight during daylight) and then refrozen (at night). The repeated melting and freezing has left the snow fairly well packed.

As the leading portion of the layer began to hang over the edge, it no longer had contact with the heat source and thus froze solid. As the next portion of the layer crept around the edge, the first portion was forced out of the vertical and toward the house wall. The process then continued. Each new portion of the ice clearing the roof edge caused the previous portions to curl more toward the house.

Notice the icicles on the end. They formed from water that had been melted by the house heat and which then drained to the edge of the snow sheet. As they began to drip from the snow, some of the water froze into vertically downward icicles. Now, however, because of the curling of the snow layer, the icicles are no longer vertical.

x4.20 Ice spikesJearl Walkerwww.flyingcircusofphysics.comAug 2010 One day, my former student Vincenzo La Salvia found this ice spike on an ice cube in his freezer, where he makes ice in an ice cube tray (rather than with an automatic ice maker).

When I was writing a monthly article for Scientific American, I received several such photos from readers. Ice spikes are rare but common enough to show in various journals. The question is, of course, how can a spike grow upward from an ice cube?

When water molecules form ice, the water must expand. If it is in an ice cube tray, it can expand only upward. The center of any ice cube slot is the last to freeze, and so the expanding periphery pushes inward and upward.

That process can form a spike if the top of the water freezes into a thin layer of ice with a small opening. Then the remaining liquid water from the center of the cube is forced out through the opening. Usually this water merely trickles off onto the top of the ice (flooding), where it freezes, but sometimes it freezes into a cone (Fig. 4-4, which is from The Flying Circus of Physics book).

If the freezing rate is slow enough, more water can be squeezed up through the cone. If it lingers at the top of the cone, the outer surface can freeze, extending the cone upward. When all the water has frozen, the cone forms a solid upward spike. These spikes are rare because their formation depends on a balance between the rate at which liquid is squeezed up through the cone and the rate at which the remaining water in the cube freezes.

In particular the spike formation depends on the beading of the liquid as it pushed up through the initial opening and then through the opening of the cone.

The beading is due to the surface tension of the liquid water --- the molecules in the liquid attract one another and resist spreading out over the top surface of the ice. This hesitation against flooding the ice allows the liquid to freeze in its elevated position, initiating the cone formation. Any chance mechanical disturbance at this point disrupts the liquid, allowing it to flood the top of the ice cube. Thus, one reason why ice spikes are so rare is that mechanical disturbance is likely during the freezing of the water. Even the slight vibration due to the freezer motor might be enough to eliminate the chance for an ice spike.

Below are my sketches of the formation of an ice spike, based on a figure published by Charles A. Knight of the National Center for Atmospheric Research in Boulder, Colorado (see the reference to his article below).

Initially the liquid water forms a slight bulge at the opening in the ice. As the point of contact of water, ice, and air moves upward, the liquid becomes more hemispherical. When the sides of the liquid are vertical, the contact point moves directly upward, giving straight sides to the spike.

A related formation is extremely rare --- the upward growth is a three sided, hollow tube. Here is my sketch based on one of the few photographs that have been published.

Knight suggested that during the formation of the spike, a chance leak allows the still-liquid water in the center of the cone to drain out, leaving only the cone. If you have the patience, you might investigate those hollow ice spikes and also the leaning ice spikes. (Why do they lean?) There is still much to be learned. The paper by Knight in the following list and the two papers published in 2004 will help your investigations. If you have any photos of ice spikes, feel free to upload them at the Facebook site for The Flying Circus of Physics:

Shortly afterwards, fern-like dendritic growths sprout from the surface, starting on the peak, as you can see in this close up.

This behavior and these images were recently published by A. G. Marin of Bundeswehr Univiversity in Munich, O. R. Enriquez of Univesity of Twente, The Netherlands, and others. (Their several publications are listed below.) Here is a link to the video posted by Enriquez that gives a microscopic view of the freezing water drop that was placed on a metal plate kept at a temperature of about -20ºC by an underlying bath of liquid nitrogen.

As Enriquez explains in the video, the freeze line between solid and liquid water begins at the contact with the cold plate and then climbs upward. We can see the freeze line because ice refracts (redirects) light differently than liquid water and thus there is contrast in the illumination above and below the freeze line. The liquid portion of the drop remains roughly spherical as the freeze line ascends because of the water’s strong surface tension due to the mutual attraction of water molecules.

However, water is peculiar in that it expands when it freezes. So, as the freeze line reaches the top of the drop, the liquid portion is pushed upward by the expansion of freezing water below it in spite of the surface tension, and then that water immediately freezes, forming a peak. That peak is said to be a singularity because the surfaces on two sides are not smoothly curved over the top but instead makes a sudden transition from side to side (the slope suddenly changes at the peak).

After the singularity forms, water vapor begins to form ice on the peak and down along the sides. The concentration of water vapor is strongest at the peak, so the formation of ice begins there. However, the ice does not form in layers. Instead it forms fern-like structures, as if miniature ferns grow outward from the drop. If you live in cold winters, you can see similar dendritic structures in the ice flowers that form on a window on very cold days. Here is one of my photographs of ice flowers that formed during a polar vortex in Cleveland, Ohio:

Michael Nauenberg of the University of California at Santa Cruz made his own pointy ice-drops but with somewhat larger drops that he placed on a block of dry ice (frozen carbon dioxide). (Obtaining dry ice is a lot easier than cooling a plate with liquid nitrogen.) He described how the freeze line forms a crater as it nears the tope. He also saw shapes other than a peak. Here is his video:

4.20 Rotating ice disk in a riverJearl Walkerwww.flyingcircusofphysics.comFebruary 2014 I am intrigued by any naturally occurring pattern; indeed I think most scientists are, and most physicists crave them. Here is a naturally occurring circular pattern that is rarely seen: A circular disk of ice floats in a river while slowly turning. That just should not be happening. Ice can be in chunks or stretched over a frozen river, but should never resemble a turntable. Years ago when I was writing the second edition of The Flying Circus of Physics I came across an article about such a rotating disk in an obscure journal. I was skeptical. There was a blurry photograph, but of course I could not see any evidence of motion.

The world has changed --- now there are lots of videos of rotating ice disks available on the web. Here are a few:

Smaller ice disks can form over places where the increased snowfall on an ice covering forces the covering downward into the water. That downward push causes water to move up through any chance opening in the covering. The water then spreads out over the ice and some of it can then freeze to form a disk that floats over the opening. The river flow beneath the covering can then stir the water in the opening, causing the disk to spin.

More interesting to me are the large ice disks, as can be seen in the videos. Such a disk forms over a whirlpool (or eddy) in the river flow. When ice chunks from upstream collect in the whirlpool, they gradually meld into a single sheet that is then rotated by the swirling water. As ice forms over the rest of the river, the rotation prevents the sheet and the rest of the ice from freezing together, and abrasion between the sheet and the rest of the ice gradually smooths the sheet into a circular disk.

Jearl Walkerwww.flyingcircusofphysics.comDec 2008 If a bottle of beer or soda is keep in a freezer for several hours and then removed before it freezes, a sudden mechanical shock to the bottle can initiate the freezing. Here is one of many videos showing the effect:

The explanation given in the video is that the mechanical shock (the tapping) releases bubbles of carbon dioxide from solution, which then causes the beer to freeze. That may not be correct. Here is my reasoning.

Beer is largely water with a small amount of alcohol and a number of ingredients such as sugar. Before the cap is removed from the bottle, the contents are under pressure (greater pressure than atmospheric pressure). Both the added contents and the higher pressure tend to lower the freezing point of the water from that of pure water at atmospheric pressure, which is about 0 degrees Celsius.

However, the beer in the freezer can be even colder than the reduced freezing point and still be liquid because water has trouble forming ice in bulk liquid. Normally, some nucleating agent such as a small solid surface is needed.

Of course, if you leave a bottle of beer or soda in the freezer long enough, dropping the temperature far enough below the freezing point, the water will eventually freeze. In short, the beer can be supercooled only so much before the water freezes.

(You definitely don’t want this to happen in the freezer because water expands when it freezes and thus the freezing will burst the bottle. You will then have broken glass embedded in beery ice, which is not easy to clean up without getting cut.)

In the videos, the bottle of beer is removed from the freezer after the beer has supercooled but before it has frozen. If you tap the bottle sharply, you cause the wall to oscillate, sending a pressure pulse through the liquid. As the pulse travels through any given part of the beer, it causes a momentary increase in pressure (which tends to lower the freezing point) and a momentary decrease in pressure (which tends to raise the freezing point). I think that the ice formation occurs during that momentary raising of the freezing point --- just then the beer is just too far below the freezing point to remain liquid and so it freezes. Once any section of the beer forms ice, the ice can act as the nucleating agent for more ice formation. In some of the videos you can see how the icy region in the beer grows until all of the beer has frozen. As the water freezes, carbon dioxide comes out of solution, forming bubbles that are trapped in the ice and which make the ice more visible.

In one of the videos a bottle of beer is opened and carried outdoors to sit on a balcony. It is not tapped but the beer soon begins to form ice, starting at the top surface. I suspect that airborne particles (perhaps snow crystals) settled down through the open top of the beer to act as the nucleating agents. Then, once ice began at the top, it acted as the nucleating agent for more ice formation, which grew downward into the bottle.

4.26 Licking pipe that is below the freezing point Jearl Walkerwww.flyingcircusofphysics.comMarch 2010 On a “triple dog dare,” one of the boys in the classic movie Christmas Story licks a metal pole that is outdoors on a day when the temperature is below the freezing point of water. He had argued that it could be done without risk and then was verbally forced to prove his point. Of course, his tongue stuck to the pole.

Although licking a subfreezing metal pole seems to be, at best, a bizarre act, several videos and news items have recently appeared about real people repeating the act. (I can only guess that their lives must be very empty.) If you are tempted to try the tongue-on-cold-pole stunt, watch this video first:

The danger in licking a subfreezing pole is obvious: the water on the tongue can freeze, which attaches the tongue to the pole. Several physics web sites correctly point out that attachment is less likely with a plastic pole than a metal pole because plastic conducts thermal energy more poorly. (Look what I am doing here: I am comparing the licking of different poles. My life must be very empty.) With a poor conductor, the thermal energy from your mouth might keep the moisture on your tongue from freezing in the contact area, but with a good conductor, that thermal energy can be conducted away fast enough that the water freezes.

If the temperature is sufficiently low, the tongue might stick to a plastic pole or any other surface (glass, ceramic) that you have a fancy to lick. Indeed, you might be able to stick you hand to a surface even though your hand is relatively dry.

To check this, I placed several items outdoors on a cold night when the air temperature was 20ºF (-6.7ºC). Two hours later, I went out to touch them with bare fingers. My fingers stuck slightly to a metal spatula and a ceramic dish. My tongue also stuck slightly to both objects. (My gosh, now I am licking things outdoors!) I checked a number of surfaces by briefly pressing a wet sponge (room temperature) to them. On metal, ceramic, and glass, small bits of the sponge were ripped off when I pulled the sponge away.

The thermal conduction argument is certainly important in explaining the effect, but a question still nags me. Why does water (liquid or frozen) adhere to a metal surface? The physics is still not understood and may depend on the actual structure of the surface. Until recently a two-layer explanation was popular. The first layer of water bonds to the metal surface by forming proton bonds or with charge transfer. Then bulk water builds up on the first layer with the normal bonding between water molecules. However, recent work reveals that the process is far more complex and variable.

I think that in the case of freezing a tongue onto a metal pole, one other factor is also important. Both surfaces are covered with nooks and crannies into which the water freezes. Once the ice forms, the surfaces are mechanically interlocked. Pulling the tongue away will break off some of the weaker nooks on the tongue surface. That is, bits of the tongue are ripped off, which is not something you want to happen. So, if you do happen to lick a subfreezing pole, you best wait for the firemen to arrive with some warm water, as in the movie.

Physics is everywhere, even in the licking of subfreezing metal poles.

x4.37 SnowrollersJearl Walker www.flyingcircusofphysics.comJan 2008A snowroller is a long, rolled up layer of snow and ice that resembles a rolled up carpet, as in this photo by Roamin'Crafter. It may be hollow along its central axis, may be 1/3 meter high, and may have a mass of 6 kilograms. So, it can be eye catching, especially if you happen to find a field full of snowrollers where it looks like a crazed person has spent the night rolling up snow. Here is one link to give you some photos. Other links are at the end of this item.http://www.crh.noaa.gov/ilx/events/roller/roller.php NOAA’S National Weather Service, brief description plus several really good photos

Snowrollers are rare but when conditions are right, many can form over a few hours. Their cause is still debated, largely because we find them after they form, not while they are forming. (Well, I certainly cannot image a life so empty that a person spends the night in a cold, snowy field on the chance of seeing a snowroller as it forms.) The best explanation is that the “right conditions” requires three things: (1) strong wind, (2) a layer of crusty snow or ice already on the field, and (3) freshly falling snow.

Snow can be crusty if many of the individual snowflakes have been melted by absorbing energy during the day (from sunlight or the passing air) and then refreezing during the night. In refreezing, the water forms a network of ice “bridges.” You can hear the bridges snap when you walk on such a network.

If fresh snow falls onto the crusty layer, the snow can stick to the icy network. If a strong wind catches any high point of the snow, the crusty layer and the overlying fresh snow can be pushed upward and then back over onto the snow, much like you would push one end of a carpet up and over to begin rolling it up.

Once the snow is rolled back on itself, it presents a larger obstacle to the wind, which can then continue to roll up the snow “carpet.” The force of the wind on the roll and the weight of the roll compacts the rolled-up snow by causing melting and refreezing. The process is the same as in making a snowball. You grab some loosely bound snow and squeeze it, forcing snow crystal to break and slide against one another. The crystals then refreeze into a denser ball, the weapon of choice in a snowball fight.

If the wind blows a snowroller in one direction, then the snowroller resembles a rolled-up carpet. However, if the wind blows it in many directions, the snowroller might then resemble an American football or a just a big snowball. In fact, you might get the illusion that Frosty the Snowman has been at play while you slept during the cold, windy night.

4.48 Heating and exploding in a microwave ovenJearl Walkerwww.flyingcircusofphysics.comJune 2013 In a microwave oven, food is irradiated with microwaves, a form of electromagnetic radiation like visible light but with a much longer wavelength. Microwaves can penetrate most foods and are absorbed by the water inside the food. The energy goes into thermal energy, so the food is heated. That is, of course, the reason we use a microwave oven. But there are certain dangers with the process, something you may have noticed if you ever attempted to heat an egg in its shell in a microwave oven. (Do not do this.) The egg will explode, making a big mess over the oven’s interior, but why? Indeed, people have put lots of things other than food into microwave ovens. (Do not do this.) The result can be an explosion that is powerful enough to blow apart the oven and thus injure, blind, or even kill someone nearby. For water-containing materials, the water can heat until it suddenly vaporizes. For other objects, the oscillating electric fields of the microwaves sets up currents that can vaporize conducting pathways or explode volatile gases.

Here is a funny video of such explosions from the British television show Brianiac. Heed their repeated warning: these are dangerous demonstrations.

Many people believe that microwaved water becomes heated because of frictional rubbing of the water molecules. However, molecules do not have surfaces and thus cannot have frictional contact. Here is the explanation for microwave cooking that I published in an earlier edition of my textbook, Fundamentals of Physics.

Electric dipoles

A water molecule is an electric dipole because the hydrogen side is more positive than the oxygen side (two charged poles, hence an electric dipole). This means that although the molecule is electrically neutral, with as much positive charge as negative charge, the slight separation of the charges produces an electric field around the molecule.

The orientation of a dipole is usually described in terms a dipole moment, which is effectively an arrow (or vector) that points from the negative side to the positive side. In the figure above, the dipole moment is marked as the "p" vector. If we place dipoles in an external electric field, the dipoles tend to line up with the field, that is, their dipole moments tend to point in the direction of the field.

First consider water that is not in such an external electric field, such as the water in a common drinking cup. The molecules are relatively free to move around but, the electric field produced by each dipole affects the surrounding dipoles. As a result, the molecules bond together in groups of two or three, because the negative (oxygen) end of one dipole and a positive (hydrogen) end of another dipole attract each other. Each time a group is formed, electric potential energy is transferred to the random thermal motion of the group and the surrounding molecules. And each time collisions among the molecules break up a group, the transfer is reversed. The temperature of the water (which is associated with the average thermal motion) does not change because, on the average, the net transfer is zero.

Microwave oven

In a microwave oven, the story differs. When the oven is operated, the microwaves produce (in the oven) an electric field that rapidly oscillates back and forth in direction. If there is water in the oven, the oscillating field exerts oscillating torques on the water molecules, continually rotating them back and forth to align their dipole moments with the field direction. Molecules that are bonded as a pair can twist around their common bond to stay aligned, but molecules that are bonded in a group of three must break at least one of their two bonds.

The energy to break the bonds comes from the electric field, that is, from the microwaves. Then molecules that have broken away from groups can form new groups, transferring the energy they just gained into thermal energy. Thus thermal energy is added to the water when the groups form but is not removed when the groups break apart, and the temperature of the water increases. Foods that contain water can be cooked in a microwave oven because of the heating of that water. If a water molecule were not an electric dipole, this would not be so and microwave ovens would be useless.

When water is heated over a flame, its temperature cannot exceed the boiling point of water because once that temperature is reached, all additional input of energy goes into forming vapor bubbles. Microwave heating is different because the microwaves are absorbed in the bulk of the water, where vapor bubbles cannot easily formed. (Surface tension tends to immediately collapse any bubble.) So in a microwave, water can superheat above the boiling point. If disturbed by motion, addition of a powder, or even addition of ice chips, the water can suddenly begin to boil vigorously, even overflowing its container and burning someone holding it.

An egg, intact yolk, or any other closed container of water will probably explode in a microwave oven. The water is heated until it flashes to vapor with a sudden outward push that bursts open the container. Sometimes the egg does not explode until it is jostled as it is being removed from the microwave oven. Then, not only does it make a big mess, but it can burn someone, perhaps on an eye surface. Smaller explosions, called microwave bumps, can occur with foods such as green beans and lima beans, which contain small amounts of water in what is an effectively closed container.

Here is a video that explains how the microwaves are generated inside a microwave oven.

February 2012 One of the popular street foods in Asiatic countries is popped rice, a fluffy, lightweight snack. The food is similar to popcorn that is common in North America and Europe. In the United States it is also similar to the Quaker Puffed Rice and Quaker Puffed Wheat, two breakfast cereals. When I was young, frequent radio advertisements would entice me with the claim that those breakfast cereals were “shot from guns.” I doubted that claim very much---why would someone be shooting grain from a gun? Now I know that the advertising claim is true. Indeed, the Asiatic street food is shot from guns, very noisy guns.

First let me explain popcorn; then we shall get to the popped rice.

Popcorn

As I explain in The Flying Circus of Physics book, popcorn is a special type of maize, grown for its ability to explode when heated by hot air or grease or when heated in a microwave oven. (In the microwave oven it heats by absorbing microwaves directly and by touching a special card that rapidly heats by absorbing microwaves.) The pericarp part of a popcorn kernel is a small, closed container of starch and liquid water. As a popcorn kernel is heated, that water partially vaporizes but largely remains liquid. Because the liquid is trapped in a container, the pressure increases and, as a consequence, so does the boiling point of the trapped water.

When the water reaches about 180°C at a pressure of about 8 times atmospheric pressure, the pericarp walls burst open, the pressure drops to atmospheric pressure, and the boiling point drops to its normal value. Thus, the water in the pericarp is suddenly well above the boiling point, and it vaporizes so rapidly that the evaporation explodes the hot, molten starch to many times its original volume. The sudden expansion against the air sends a sound wave through the air—the pop of the corn.

Popped riceRice carries a small amount of water but it cannot act as a container of the water if the rice is heated in, say, a microwave oven. Heating simply evaporates the moisture out of the rice grains, with no expansion of the grains.

The street vendors use a different, far more dramatic technique. Here are several video examples:

The idea is to seal the rice in an airtight metal container that is then heated. The container is rotated so that the rice is mixed and evenly brought to a high temperature. As the air trapped in the container increases in temperature, the air pressure also increases. With that higher air pressure, the temperature at which water will vaporize is increased. The result is that the water in the rice grains remains in liquid form in the grains.

Then, once the rice and trapped air are very hot, the vendor releases a valve that suddenly allows the trapped air to escape. It explodes outward. Because that air must do work to push its way into the surrounding air, it needs energy, but the expansion is too rapid for the energy to come from anything but the air itself. Thus, the air gives up some of its internal energy (associated with the random motion or thermal motion of the air molecules). Such an expansion is said to be an adiabatic expansion. The result is a rapid drop in air temperature and also the boiling temperature of water. The water dramatically vaporizes, which greatly increases its volume. It blows the rice grains out to a much larger size --- the rice is now the fluffy popped rice that the vendors sell.

It is propelled by a discharge of water from the two pipes that run from a “boiler” to the rear of the boat. (The boiler could be merely several turns of tubing.) To prepare the boat, you fill the boiler and pipes with water, float the boat in a pool of water, and then place a lit candle beneath the boiler. As the water in the boiler heats and evaporates, the increased pressure shoves the water in the pipes out to the rear of the boat.

The curious feature is that once the water is discharged, the propulsion does not cease. Instead, the boat is pushed forward every few moments in a stutter-step fashion. When water is expelled from the pipes, some of the steam generated in the boiler moves into the pipes, where it condenses because of the cooler environment. The movement and condensation both act to lower the pressure in the gas. As the pressure falls, water from behind the boat is pulled into the pipes, refilling them. Then the whole cycle of discharge and refilling repeats itself, with the boat being shoved forward again.

The rapid expulsion of water is in the form of a jet directed toward the rear, which requires that the boat move forward. The boat does not move backward during the refilling stage because the water intake is not a jet. Instead it is slower and from a wide range of angles (approximately a hemisphere). Thus, the force tending to pull the boat backward is weak and cannot overcome the water drag on the boat. So, the boat moves forward during each discharge but is stationary during each refill.

One of my putt-putt boats is powered by a candle (the upper one in the photo), and another is powered by cooking oil. In that second one, a short wick extends through a central hole in a metal disk that floats on a small container of the oil. I coat the wick with the oil and then light it. As the oil is vaporized and then burned, the wick pulls more oil up into it to continue the burning process.

To set up the whole process, I first pour water into one of the pipes until it runs out of the other pipe. Then, with boat angled so that the water does not pour out of the pipes, I lower the boat onto the water in a bathtub and light the wick in either the candle or cooking oil. After a few minutes, the boat begins to chug across my bathtub.

To see me demonstrate a putt-putt boat to Jay Ingram, the host of the Daily Planet television show on Discovery Channel Canada, go to the Facebook site for The Flying Circus of Physics and click on the video posted January 1, 2011 and called “The putt-putt boat.”

4.54 Collapse of railroad storage tankJearl Walkerwww.flyingcircusofphysics.comFeb 2009 Railroad tank cars are extremely durable and normally can be damaged only in a high-speed crash. However, they can also be ruined when certain principles of physics are ignored. Here are two links to the same video and a separate set of photos (taken in a different situation). In the videos and the photos you see a tank car that is crushed as if some giant creature from a grade B science fiction movie has stepped on it during a rampage. What could cause such destruction of something so strong?

The two links show a staged demonstration that was meant to be a warning, but the photos show a very embarrassing accident that was caused by a cleaning crew. The crew had been using steam to clean the interior of the tank car but was unable to finish the job by the end of the day. So, they turned off the steam, sealed the vents on the car, and left for the night. Sealing the vents was the big physics error.

When it was being cleaned, the tank car’s interior was filled with very hot steam, which is a gas of water molecules. When the crew closed the vents and left for the night, the car was still filled with steam. At that point the pressure of the gas in the car was equal to atmospheric pressure because the valves had been opened to the atmosphere during the cleaning.

As the car cooled during the night, the steam also cooled and much of it condensed, which means that the number of gas molecules and the temperature of the gas both decreased while the volume of the gas (the volume of the car’s interior) was constant. As a result, the gas pressure within the car decreased. At some point during the night, that gas pressure reached such a low value that the external atmospheric pressure was able to push the car’s steel walls inward, crushing the car. The cleaning crew could have prevented this accident by leaving the valves open, so that air could enter the car to keep the internal pressure equal to the external atmospheric pressure.

Teun van Heesch, a physics teacher in the Netherlands, sent a news item about a similar accident in which two silos were crushed by atmospheric pressure. Presumably, the pressure inside the silos was not matched to the external pressure as the air temperature fell and frost formed. The third silo that you can see in the photo here survived because it was filled with pieces of polystyrene. The news story is reproduced down below here, after the references.

Similar crushing physics has long been used in physics and chemistry classrooms but usually without the internal water vapor. Instead, an open can is heated by a flame and then sealed. As the air inside cools, the pressure decreases to the point where the external atmospheric pressure can crush the can. Here are several videos.

4.61 Pub trick --- cooling beer on a hot dayJearl Walkerwww.flyingcircusofphysics.comNov 2010 Let’s imagine that we are in a pub’s outdoor patio on a very warm day. We have plenty of bottles of beer but the beer is at air temperature, which means that the beer is not as enjoyable as when it is chilled. So, we would like to decrease the beer temperature as much as possible. However, for various reasons (such as rowdy behavior), we are not allowed to enter the main section of the pub to cool the beer in a refrigerator or to gather ice. All we have in the outdoor patio are chairs, tables, cloth napkins, plates, forks, spoons, knives, remains of our lunch, decretive rugs, decretive unglazed pottery, dirt, sand, plants, and a limited amount of shaded ground. Of course we already have the beer in the shade to product it from the chemical changes caused by direct sunlight. (The light produces 3-methyl crotyl mercaptan, which gives the beer a “skunky” taste and smell.) But is there anything else we can do?

The native people who inhabited the hot regions of North America (and many other people who lived in other hot regions) knew a way to cool their food. The idea is to place the food in a porous container and then wet the container so that the water gradually evaporates from the container wall into the external air. That evaporation requires a fair amount of energy, which comes from the contents of the container. Thus, the food is cooled and then kept cool as long as the container is wet and provided that the air humidity is low.

Energy is required for evaporation because in the liquid state, water molecules are electrically bound together. They are not tightly bound as in ice, but loosely bound so that they form fleeting groups of two or three molecules. In order for water to evaporate, energy must somehow be provided to free the molecules from one another. On a stove, the energy can come from a flame. In bright sunlight, the energy can come from the sunlight. In the shade on the pub patio, the energy can come largely from the interior of the container.

If the food starts out cold (as butter would if it comes from a refrigerator), the container can keep it cool for a long time. If the contents start out at an air temperature that is warm or hot, the cooling would be only gradual. You won’t end up with a cold beer just above the freezing point of water, but it will at least not be hot.

You can improve the design by placing a smaller pot inside the larger one and by placing wet sand between the two. That way the outer pot remains moist without close attention. To decrease the chance that thermal energy enters the pots from the hot ground, the outer pot can be elevated or placed on moist sand. To prevent food inside the smaller pot from becoming moist, you can use a glazed inner pot.

The image here shows a butter container that I have used for years and which was described in my old Scientific American article about various kitchen gismos. About 15 minutes before I need the container, I invert the top portion and fill it with water. After the water has soaked through the wall, I pour out the remaining liquid and then place the top portion down over the butter in the bottom portion. The container keeps the butter cool for several hours and is almost cold to the touch with my fingers.

Here is a wine cooler that works on the same principle. I fill the cooler with water, let the water soak through the wall, drain the water, place a wine bottle inside, and then cover it with a dry cloth. The bottle is cooled within about 30 minutes or so, depending on the evaporation rate from the clay.

Here is a drawing of a janata, a similar evaporative cooling container found in India. Note that the smaller pot sits in the water that is in the larger pot, and the two pots are covered with a damp cloth.

And here is an image of a zeer, an evaporative cooling container found in Nigeria, Sudan, and other African countries. Note the sand between the smaller and larger pots. Vegetables that normally would spoil within days if kept at room temperature in those parts of Africa can last several weeks in a zeer.

4.78 Pub trick --- lighting a candleJearl Walkerwww.flyingcircusofphysics.comMarch 2011 The challenge this month is to light a candle with a match. Ah, that is easy, you say. Well, the challenge is actually to light a candle with a match without putting the wick directly in the flame of the match.

Try it. Even if you hold the match near the wick, the candle will not light.

The trick is this: Light the candle in the normal way and then blow it out as you hold the still burning match. Once the flame on the wick is extinguished, hold the flame of the match above the wick. The candle relights.

Normally a candle emits light because the flame on the wick melts and then vaporizes the wax in the candle. As the wax vapor ascends into the flame, the wax particles are ignited and emit light. So, the light comes from the burning vapor, not the burning wick.

Right after you blow out the candle, hot vapor still rises from the candle. There just is no longer any flame to ignite those wax particles. But if you hold the match flame in the stream of upward moving wax particles, they ignite in the match flame, which then causes a chain reaction of ignition all the way back down to the wick. Thus, you light the candle without putting the wick directly in the flame of the match.

The same trick can be seen in these two videos (the first is the set up and the second is the answer):

4.80 Revenge of the turkeysJearl Walker www.flyingcircusofphysics.comNov 2007 Do you want to be on the local 5:00 news? Here is a sure-fired way. Cook a turkey in a hot-oil bath, as has become a minor rage in the United States. The apparatus consists of an upright cylinder containing peanut oil, which is heated to 177ºC (350ºF) by a propane flame beneath it. The turkey is lowered into the oil and allowed to cook for about 3 to 5 minutes per pound, until it reaches a temperature of 77ºC in the breast. Then it is pulled out of the oil, allowed to drain for about 30 minutes, and then served.

What could be dangerous about that? Here’s what. If you neglect to thaw the turkey from its frozen state or if you forget to dry off the thawed turkey, you can easily set the place on fire with burning oil. If you do all this indoors, you’ll be eating dinner in the local shelter after the fire personnel salvage what they can of your house. In short, you will be the victim of turkey revenge.

You already know the physics behind the turkey revenge if you have ever deep fried a basket of frozen French fries (chips in UK language and maybe just potato slices or American fries in France). The French fries contain frozen water, both on the surface and in the pores along the surface. When the oil in the hot oil bath reaches any such water, the transfer of thermal energy from the oil to the water is so sudden that the water flashes to steam, which requires that it greatly expand its volume. Thus, along the French fry surface, water suddenly pushes outward on the surrounding oil, and then bubbles of water vapor rise rapidly from the surface because the water vapor is much less dense than the surrounding heavy oil. When a bubble reaches the air, the oil over the top of the bubble quickly drains and thins until it bursts, slinging out oil drops and sending waves over the oil bath. Frying like this is usually safe in a restaurant because the oil bath is wide enough to catch the oil spray.

The cooking apparatus for the turkeys is not as wide. Indeed, the cylinder is barely wider than the turkey so as to minimize the amount of peanut oil that is required for the turkey to be submerged. If the turkey is frozen or still wet with liquid water on the skin, the sudden production of water-vapor bubbles causes the oil to be thrown from the cylinder or to pour over the side. The oil is ignited if it pours down onto the propane burner. If it also pours over the surface supporting the burner, that surface is also ignited. And then … well, you can complete the sequence.

Putting a wet or frozen turkey into a hot oil bath is a wonderful lesson in the physics of phase transition (from the solid or liquid phase to the vapor phase). It also beautifully illustrates the fact that the transition to the vapor phase requires the molecules to move apart so that they are no longer bound together as in the solid phase or even in transient associations as in the liquid phase. The mechanics of bursting bubbles on the surface is still open research. And the speed at which a flame can travel across a pool of hot oil still fascinates physicists. You and the fire personal will have lots to talk about as you roast marshmallows over the smoldering ruins of your house.http://whatscookingamerica.net/Poultry/CajunFriedTurkey.htm Plans about cooking a turkey in hot oil

If you light a bonfire, the flames normally just rise along chaotic and inconsistent paths. But in this Australian fire, the influx of cooler air into the burning material developed into a vortex, often called a fire devil (as opposed to a dust devil). So, the incoming air not only comes in radially but also toward one side of the fire, to establish a circular flow to the rising, burning gases. The vigorous influx of air causes the fire to burn more fiercely, which in turn causes a greater influx of air. The situation is exactly what the firefighters did not want, but as explained in of the videos, the sight was so intriguing that the firefighters stopped and watched for a while.

4.84 Warmth of greenhouses, children dying in closed carsJearl Walkerwww.flyingcircusofphysics.comOctober 2014 Why is a greenhouse relatively warm? Does it have a special type of glass that somehow traps thermal radiation (infrared radiation)? Why does the interior of a closed car become hot if the car is left in direct sunshine on a hot day? Very sadly, here in the United States we have had many cases where a child has been left in such a closed car, resulting in several deaths. After one of those deaths made national news, this next video was made to demonstrate how quickly the temperature inside a closed car can increase even in a moderately warm outdoor temperature.

The primary reason that a greenhouse is warm is that the enclosure cuts off or severely limits air circulation. Thus, warm air is not allowed to rise out of the greenhouse to be replaced by cooler air flowing along the ground; also breezes are not allowed to displace the internal warm air. (A common myth is that the glass or plastic roof of a greenhouse somehow traps thermal radiation. Unfortunately, because the term greenhouse effect is often applied to the trapping of thermal radiation by Earth’s atmosphere, the idea of such trapping is erroneously carried over to a greenhouse.)

A closed car parked in direct sunlight on a hot day is like a greenhouse. Its interior can become very hot, because air circulation is eliminated. In fact, if the sunlight enters through the front windshield, the dashboard and steering wheel can become hot enough to burn skin. Establishing circulation by lowering the windows or opening the doors can (slowly) decrease the temperature. Because black paint absorbs visible light more readily than white paint, you might think that a black car would be hotter than a white car. However, heating a car is primarily due to the absorption of infrared radiation, not visible light, and the two paints probably absorb about the same in the infrared range.

4.89 Energy in a heated roomJearl Walkerwww.flyingcircusofphysics.comSeptember 2014 Suppose that you return to your chilly dwelling after snowshoeing on a cold winter day. Your first thought would be to light a stove. But why, exactly, would you do that? Is it because the stove would increase the store of internal (thermal) energy of the air in the dwelling, until eventually the air would have enough of that internal energy to keep you comfortable? As logical as this reasoning seems, it is flawed, because the air’s store of internal energy would not be changed by the stove. How can that be? And if it is so, why would you bother to light the stove?

A dwelling is not airtight (in fact, an airtight dwelling would not be safe). As the air temperature is increased by the stove, air molecules leave through various openings so that the pressure inside the dwelling continues to match the atmospheric pressure outside the dwelling. Although the kinetic energies of the remaining molecules increase, the total kinetic energy does not increase because fewer molecules are in the dwelling.

So why does the dwelling feel more comfortable at the higher temperature? You have a tendency to cool because (1) you emit infrared radiation and (2) you exchange energy with air molecules that collide with your body. If you increase the room temperature by lighting the stove, (1) you increase the amount of infrared radiation you intercept from the surfaces in the dwelling (walls, ceiling, floor, furniture, etc.), replacing the energy you lose through infrared radiation, and (2) you increase the kinetic energy of the colliding air molecules and you gain more energy from them.

When I put this subject into my textbook (Fundamentals of Physics by Halliday, Resnick, and Walker), I used it as a Sample Problem. The solution goes this way:

The internal energy Eint of the air in the room is related to the air temperature T by

Eint­ = nCVT,

where n is the number of moles of air and CV is the molar specific heat at constant volume. We want to find the change ΔEint of the internal energy when the temperature is increased. Because both n and T can change, we write

ΔEint = Δ(nCVT) = CV Δ(nT).

We can replace nT by using the ideal gas law in the form

pV = nRT,

where p is the air pressure, V is the air volume (the room volume), and R is the ideal gas constant. We then have

ΔEint = CV Δ(pV/R).

Because the pressure p and the volume V are constant, the term within the parentheses is constant and thus there is no change.

ΔEint = 0.

The internal energy of the air in the room does not change in spite of the increase in temperature.

4.91 A radiometer toy and its reversalJearl Walkerwww.flyingcircusofphysics.comAugust 2014 A radiometer was a device invented in 1872 by William Crooke to measure the energy emitted by a light source, but today it is a novelty or toy sold in science shops. Inside a sealed, partially evacuated glass bulb, four vertical metal vanes are attached to a metal hub that can rotate around a vertical needle. The vanes have the same arrangement of colors: white on one side and black on the other side.

When the device is mounted near a light source, the vanes and hub rotate around the vertical needle, rotating faster for brighter light. What causes the rotation, what is its direction (does, for example, the black side of a vane lead), and how can it be reversed?

The motion is often attributed to the pressure of light, but that effect is far too small to observe with the toy and, besides, it would yield a rotation that is opposite what is seen. Here is the argument: Light can push on object, and the push is greater if the light reflects from the object. Thus, light shining on the vanes will push on the white sides more than the black sides, and the vanes should rotate with the black sides leading. Were the bulb almost fully evacuated, the vanes would indeed turn like this.

However, the pressure on the vanes due to the residual air gives a much larger effect. Because light (infrared radiation and visible light) is absorbed more on the black side of a vane than on the white side, the black side becomes slightly warmer than the white side. Because the residual air molecules run into a vane, they push on the vane. The faster the molecule is moving, the greater the push. The air molecules on the black side of a vane move faster than those on the white side because of the temperature difference. Thus, the push on the black side is greater than that on the white side, and the vanes rotate around the support pin with the white side leading. After a while, the two sides of each vane reach the same temperature (they reach thermal equilibrium), and the effect disappears and the vanes stop rotating.

To reverse the motion, put the toy in a refrigerator. The black side of each vane loses thermal energy slightly faster than the white side via infrared radiation, and so the white side then has the higher temperature and the greater push from the air. Again, the rotation continues until thermal equilibrium is reached.

The paper by Jane Wess, listed below, is a delightful review of various (some quite curious) radiometer designs.

4.94 Rubber bands and the direction of time, me in entropy trouble at MITJearl Walker www.flyingcircusofphysics.comOctober 2006 Inflating a balloon with your breath and stretching a rubber band with your hands require effort because the rubber (or rubber-like material) resists being stretched. In most materials, the resistance to stretching is due to the forces that bind the atoms and molecules together. Because any stretching tends to separate the atoms and molecules, the binding forces resist the stretching. However, rubber is very different because it is elastic and, for small extensions, the stretching does not tend to increase the separation of the atoms and molecules. Thus, its resistance is not due binding forces. What causes a rubber band or balloon to resist stretching? Rubber consists of cross-linked polymer chains (long molecules with cross links) that resemble three-dimensional zig-zags. When the rubber band is at its rest length, the polymers are coiled up in a spaghetti-like arrangement. Because of the large disorder of the molecules, this rest state has a high value of entropy, the measure of disorder. When we stretch a rubber band, we uncoil many of the polymers, aligning them in the direction of stretch. Because the alignment decreases the disorder, the entropy of the stretched rubber band is less. Thus, the force on our hands from the rubber band is due to the tendency of the polymers to return to their former disordered state and higher value of entropy. Because entropy naturally increases in the world, it is often said to give direction to the "flow" of time. That is, the flow is in the direction of increased disorder. For example, the molecules that bring the stench of a skunk to you (as time flows in the "proper" direction with the molecules spreading out) will not naturally recollect at the skunk (as if time could be reversed like a video run backwards). Thus, the force on our hands from a stretched rubber band is related to the direction of time. When I was at MIT in Boston, I lived in the East Campus Dorm, which consists of two parallel buildings. When spring arrived, the buildings would fight each other with giant slingshots made of surgical hose. Typically, the window in a dorm room was removed and then two lengths of the hose were fastened to opposite sides of the window frame. The lengths of hose were connected to a pouch that was pulled with great effort almost to the hallway. After a water balloon was fitted into the pouch, the pouch was released and the rapidly contracting hose hurled the water balloon at the opposite building of the dorm. If we were lucky, the water balloon would hit a window on the opposite building, crashing through the glass and into the room, spraying water everywhere. This was especially fun when we added dye to the water, to color the other room. "Festive," I thought. Now, you might think that we were an unruly bunch. However, what we were really doing was studying how the force due to entropy can result in the mechanical energy of a projectile. At least, that is what we were going to tell the campus police if they ever caught us. However, we were always able to dismantle and move the slingshot before the police could pinpoint our position.

4.95 Tunnel firesJearl Walker www.flyingcircusofphysics.comJanuary 2007 I love crawling through caves but I hate driving through tunnels. The big difference is that in a cave I am in control but in a tunnel I depend on the wisdom (and sobriety) of all the drivers around me. My big fear is a crash that results in a fire, which happens fairly often somewhere in the tunnels around the world. Being trapped in a long tunnel by a fire with billowing smoke is the stuff of nightmares. Let’s see if physics can offer any guidance on what to do in such a situation.

Because hot gas from a fire is less dense than the surrounding cooler air, the hot gas rises. In an open area, the hot gas would probably reach a considerable height but in a tunnel, the ceiling blocks the gas. So, the gas spreads in both directions along the ceiling. The motion creates an air flow: clear air moves along the roadway toward the fire, gets caught up in the burning, rises to the ceiling, and then spreads along the ceiling. Thus, near the fire (within 50 m to 100 m), someone in a car or on foot may be able to avoid the smoke by being in the inflow of clear air. However, farther from the fire (200 m), the hot gas has cooled somewhat and become turbulent, and it begins to mix the smoke with the road-level air. This road-to-ceiling smoke front moves along the tunnel and may be dense enough to be life-threatening. Indeed, the smoke in a tunnel fire is the primary danger for anyone not involved in the crash that caused the fire. The smoke front moves at about 1.5 meters per second, which is about the pace of someone walking quickly. Thus, if people are trying to walk away from the fire, they can easily be overtaken by the smoke front.

Modern tunnels, especially long ones, are ventilated by an air flow of about 3 meters per second in the direction of the traffic flow (if the traffic flow is one way). The idea is not only to remove car exhaust fumes but also to provide a margin of safety in case of a fire. The speed of the air is chosen to prevent the smoke from moving upstream (against the air flow and thus also against the traffic flow). Motorists downstream should be able to escape from the tunnel by driving out of the tunnel but motorists upstream cannot easily back up out of the tunnel. The air flow is designed to give the upstream motorists a chance of survival by preventing the smoke from reaching them. There is, of course, a flaw in the plan. If the downstream motorists are caught in a traffic jam and cannot drive out, then the smoke might easily overtake them because the smoke front is pushed along by the ventilation flow.

4.96 Frost circles and the danger of sinkholesJearl Walker www.flyingcircusofphysics.comJanuary 2007 During certain weather conditions, in certain parts of the British Isles, frost-free circles will appear in regions that are otherwise covered with frost. More than just a novelty, these circles (called frost circles in spite of the fact that they are frost free) are warnings that the underlying ground might be unstable and subject to collapse. Occasionally, the ground does collapse to form a sinkhole large enough to swallow up a car, large truck, or even part of a house. What causes frost circles?

Answer The British Isles, California, and many other locations have many abandoned mines, often with little or no above-ground evidence of their presence. Gas, such as methane, can escape from the shafts of such a mine by flowing up through the ground to the surface. The gas may already be warm and might become warmer by oxidation once it reaches the surface. Either way, it can warm the region surrounding its escape point by several degrees. If frost or snow covers the ground, this warming can clear the ground in a circular region around the escape point.

The effect is most noticeable during changes in barometric pressure. If the barometric pressure is high for several days, air is forced down into the mines, either through mine openings, chance cracks, or even the overlying ground. Then if the barometric pressure drops, the pressure in the mine is then higher than the external pressure, and some of the mine gas is pushed outward.

Regions in which mine gas escapes may indicate that the ground overlying the mine is relatively thin. Weathering or the presence of a sudden heavy load on the ground might cause such a thin ground layer to collapse to form a sinkhole. When this occurs in an urban environment, a truck driver might suddenly be headed nose down into a pit or a homeowner might open up the front door to find that the front lawn is no longer there. Thankfully, slumping of the ground is more common than the sudden formation of a sinkhole. If the ground slumps, don’t drive heavy machinery over the top of it!

4.97 Cooking on sidewalks and dashboardsJearl Walker www.flyingcircusofphysics.comAugust 2007 A common complaint on very hot days in the United States is to say, “It is hot enough to cook an egg on the sidewalk!” Is the statement an exaggeration, or is cooking on a hot sidewalk actually possible?

In the links that follow, you can watch several people test the statement. They crack open an egg and try to cook it on a skillet or aluminum foil that they place on a concrete sidewalk or street; sometime they put the egg directly on the concrete. Usually the egg stiffens up as it dehydrates, but it never cooks except in one video in which the egg is placed on a steel panel that has been left lying on a desert ground in Jordan. There the egg soon begins to cook, first along the perimeter of the egg white.

Cooking an egg means that you change the nature of the proteins so that they can bond to form a network that traps the water in the egg. A sunny-side-up egg is solid because of the network, yet still moist because of the trapped water. The protein transformation begins when the egg temperature reaches about 140ºF (about 65ºC), which is much higher than what we see in the videos except the one shot in Jordon where the desert ground temperature can exceed 140ºF.

The people trying to cook an egg on a sidewalk make a common error. They should switch to something that readily absorbs sunlight, like a blacktop road. Concrete reflects fairly well in the visible region and also in the infrared region, but you want both regions to be absorbed if the improvised cooking surface is to heat up appreciably in the sunlight. When I grew up in Texas, I learned the hard way that I might be able to run barefooted over a concrete sidewalk without harm but never over a blacktop road, where I could be severely burned. I never thought about cooking an egg on the blacktop roads, but there were always plenty of roadkill cooking on them (and they did not look too appetizing and smelt ever worse).

The Texas summers also taught me to never climb immediately into a car that had been closed up and parked in direct sunlight. The car could feel like a furnace, and the steering wheel, dashboard, gear control, and seats could burn skin. The interior was much hotter than either the surrounding air or any other surfaces in direct sunlight. It was even hotter than the objects in the trunk of the car. So, how does, say, the dashboard become so much hotter than an external object that is exposed to the same direct sunlight?

The difference is that the dashboard is in an air-tight enclosure when it is exposed to the sunlight. If the dashboard were outside the car or even in a car with good ventilation, much of the thermal energy delivered via the sunlight absorption would be transferred to the surrounding air. As air warms up, its density decreases, and so the hot air is pushed upward by a buoyancy force due to the somewhat cooler and denser air that surrounds it. This continuous transfer of energy to the air flow keeps the temperature of the external dashboard from rising high above the overall air temperature. If a breeze blows across the dashboard, the transfer of thermal energy to the air is even faster and the sndashboard temperature probably matches the overall air temperature.

In a parked closed car, however, the air is trapped and cannot carry thermal energy away from the dashboard, and there is no breeze. So, as the dashboard continues to absorb light, it heats up until it is hot enough to cook an egg … or cake and cookies as you can see in some of the videos.

4.98 Baked Alaska and frozen FloridaJearl Walker www.flyingcircusofphysics.comDecember 2007 Bake a cake (a layer of cake) of almost any type until the center is springy to the touch. While the cake cools, separate egg whites from the yolks in three eggs and then whip the whites to make a meringue. Frequently check the consistency by lifting the beater from the whites. When little peaks are left where the meringue last clings to the beater, gradually beat in ½ cup of sugar.

By now, the cake should be cool. Place a layer of hard-frozen ice cream on top, in the middle of the layer, and then cover it with a layer of meringue. Next, bake the arrangement in an oven at about 230ºC (450oF) for about five minutes or until the meringue browns. Then immediately serve the desert. The photo here, by Tiffany Ernst, shows a typical baked Alaska in cross section.

Although this desert consists of just common cake, ice cream, and meringue, it is considered to be festive largely because it is such a novelty in contrasting temperatures: warm or hot on the exterior, cold in the interior. And the common desert connoisseur is amused by the question: how can you bake ice cream without melting it?

The reason is, of course, the meringue. When you beat the egg whites, you are unraveling proteins and allowing them to crosslink to one another to form a mesh. You are also continuously adding air that becomes trapped in the mesh, giving the meringue its light, fluffy texture, part of its appeal. Well, any layer with a lot of trapped air is a poor conductor of heat. Thus, the meringue acts like a wool coat on you or a fur coat on an animal, insulating against any heat transfer. However, in the case of the desert, the layer acts to minimize the inward transfer of thermal energy, not the outward transfer as with a coat.

A frozen Florida (also known as an inverted baked Alaska) is a desert with a hot interior and a cold exterior. Here are two versions. The first is from Nicholas Kurti, who coauthored the book But the Cracking is Superb with his wife Giana Kurti. A container is fashioned out of meringue and ice cream, and then a blend of apricots, syrup and apricot brandy is poured inside and covered with more meringue. Then the container is placed in a freezer for several hours. Finally, just before it is to be served, the desert is heated in a microwave oven for several minutes. The microwaves are absorbed much more by the liquor than by the meringue, and so the result is a hot interior surrounded by a cold meringue.

Here is another version, reported by Kerry Parker in the article listed below and attributed to Peter Barham who wrote the delightful book The Science of Cooking. Encase a glob of sugary jam with a covering of ice cream and then microwave the structure. The microwaves heat the jam much faster than the ice cream. As mentioned in The Flying Circus of Physics book, jam readily absorbs microwaves, as you might have noticed if you have ever made the serious error of biting into a pastry with a jelly interior after you microwaved the pastry. (You can very badly burn your mouth.)

Microwaves are readily absorbed by liquid water because the oscillating electric fields of the microwaves force the water molecules (which are electric dipoles) to rotate as they vainly attempt to line up with the electric fields that continuously change directions. Wherever the molecules happen to form fleeting groups of three, the rotation can break the bonds among the molecules, thus putting energy into the liquid. This energy quickly ends up in the thermal motion of the liquid; that is, the liquid warms.

Such grouping and the breaking of bonds is possible in liquid water but not in ice, where the molecules are locked into place in the crystalline structure. Thus, ice absorbs microwaves only poorly. You can, of course, thaw frozen foods in a microwave, but you depend primarily on remaining pools of liquid water in the foods where concentrated substances have lowered the freezing point below the lowest temperature obtained by the freezer.

Similarly, the alcohol, sugar, and other materials in the water within the interior of a frozen Florida prevent the interior from freezing when the desert is placed in a freezer. Then, when the desert is heated in a microwave oven, the still liquid water in the interior readily absorbs microwaves. However, the meringue and frozen ice cream shell contain only small pockets of liquid water and absorb microwaves much less. Therefore, after a minute or two, the interior can be hot and the exterior can still be cold. Kerry Parker suggests that you might need to experiment to find the best power setting and heating time. Let me know what you find and I’ll post the results here and on the Flying Circus blog site.

· Parker, K., “Learning about insulation and the flow of heat never tasted so good,” Physics Education, 26-27 (January 2004)· Kurti, N., “The Physicist in the Kitchen,” Royal Institution of Great Britain Proceedings, 42, 451-467 (1969); a brief description of this article is found at http://khymos.org/history.php

4.99 A cartoon, MIT, and cornstarchJearl Walkerwww.flyingcircusofphysics.comFeb 2010 A recent Doonesbury cartoon by Gary Trudeau demonstrated that flirting by MIT students can be different. Well, that is not the right word. Quirky is the right word. It certainly was quirky when I was a student there and tried to talk to the BU women at the mixers. Let’s just say that mixing does not come easy to an MIT student.

When the cartoon appeared, two people immediately sent me a copy of it or a link to it, a non-subtle way of telling me that I still do physics when eating with a group of people. For example, I examine the optics of a glass of water, the clinging nature of the gravy on my plate, the melting rate of some ice cream, and the direction in which Guinness bubbles move when the beer is poured. I also often end up with a spoon hanging from my nose to explain how a thin film of water can act as adhesive. So, the cartoon is me.

Let me quickly explain the physics in the cartoon. You probably know the egg trick since it has been around for at least a century. When the flame heats the internal air, the increased thermal energy of the air molecules causes the air to expand, pushing its way out of the bottle by flowing through the tiny gaps between the egg and the bottle’s lip. When the flame is out, the air remaining in the bottle loses its extra thermal energy to the bottle’s wall. As the thermal energy decreases and the air molecules slow down, the air pressure drops. The internal air pressure is then less than the external air pressure. The pressure difference presses the egg down firmly onto the bottle’s lip, preventing air from flowing into the bottle. Eventually the pressure difference is enough to push the pliable and slippery egg into the bottle, and then air pressures can equalize.

The other trick alluded to in the cartoon involves a slurry of cornstarch and water. If you want to read my 1978 article about the strange behavior of the slurry and various other fluids (including Silly Putty, ketchup, and certain hair shampoos), go to The Article of the Month section of this web site. The link for a quick jump is given below, but here is a quick summary of the physics.

The cornstarch slurry is an example of a non-newtonian fluid. Most of the fluids you encounter, such as water, milk, soda, and beer) are Newtonian fluids. They each have a viscosity, which a measure of their internal friction against flowing. Water has low viscosity and easily flows. Honey has high viscosity and is sluggish in flowing. The viscosity can be changed by changing the temperature. For example, cold honey is more viscous than hot honey. The temperature effect is also true of non-newtonian fluids but their viscosity can also depend on the pressure put on them (stress) or the attempt to sliding one layer over another (shearing). For example, if you put a large pressure on the cornstarch slurry, it becomes rigid and refuses to flow. As soon as the pressure is relieved, the rigidity disappears and the slurry’s viscosity returns to its initial value.

This means that if you slap the cornstarch slurry, it does not splash because it is rigid in the contact region with your hand. When I toured with the Flying Circus of Physics talk, I would demonstrate this strange effect, showing how readily the slurry flows before and after I hit it. However, if the slurry had too much water, it splashed just fine when I hit it, throwing the slurry all over me and the first row of the audience. People who had already seen the talk knew to sit well away from me for my first attempt. Only the second attempt, with more cornstarch in the mix, worked correctly.

As explained in a recent paper by Bischoff White, Chellamuthu, and Rothstein of the University of Massachusetts, the cornstarch slurry becomes rigid under stress because the stress causes the particles to jam together as logs on a river do in a log-jam. Elsewhere in this web site, I talk about the physics of a cornstarch slurry and give a link to my all-time favorite video, in which someone attempts to run across a vat filled with a cornstarch slurry. The idea was to move rapidly across the slurry, slapping down a foot to make the slurry rigid and then pulling the foot back up as the rigidity disappeared and the foot began to sink. This is easily the funniest video in all the thousands of video links I have here at the FCP site.

4.101 Cell phones popping popcornJearl Walkerwww.flyingcircusofphysics.comMarch 2010 For the last several years, videos such as this one have gathered huge numbers of views, partly because they are just unbelievable and partly because they are a bit scary. Several cell phones (mobiles) are positioned around popcorn kernels. Soon after the phones are activated, the kernels explode into popcorn.

Science teaches us to make observations and draw reasonable conclusions. Let’s use a few observations here. When you use a cell phone, with the device pressed against your ear, does your ear heat up and threaten to explode? Do you feel even the slightest bit of heating? If you are confronted by a gang of ruffians on a deserted street, can you simply aim your cell phone at them to overheat their eyes? If you are caught outdoors in a blizzard, can you rub your cell phone over your body to keep from freezing to death?

No, of course not. So, if an object as big as your ear intercepts the transmissions from a cell phone without any sensation of heating, how could a small kernel intercept so much that it explodes?

Popcorn is a special type of corn in which the pericarp section contains a small amount of liquid water. When the kernel is heated, the water temperature increases well beyond the normal boiling point of water because the water is trapped. When the water temperature reaches about 180ºC, the water suddenly vaporizes, creating such a large increase in pressure that the kernel explodes to many times its original volume. The explosion produces the audible pop for which popcorn is named.

So, in order to produce that sudden vaporization of the trapped water in a kernel, the transmissions from a cell phone would have to increase the water temperature to 180ºC, a temperature at which you can cook meat and vegetables. Those transmissions are a form of electromagnetic waves. You, your cell phone, and any popcorn kernels you happen to have are already bathed with electromagnetic waves radiated from the communications towers that carry cell phone messages. The antenna in your cell phone sends a signal to the local towers so that the cell phone system knows where you are. When you answer a phone call, the antenna sends out a series of electromagnetic pulses that encode your voice or text message. Water molecules can absorb some of those pulses just like water in food can absorb electromagnetic waves in a microwave oven, to heat and cook the food.

Your ear contains water that absorbs some of the waves from the cell phone antenna. However, the absorption is so small that you are unaware of it. In fact, you would experience a greater increase in the temperature of your ear if you walked outdoors on hot, sunny day. Thus, a cell phone cannot possibly heat the water in a popcorn kernel to a blistering 180ºC.

Still, there are several videos that seemingly show cell phones popping popcorn or cooking an egg. Here is one that pretends to show four cell phones cooking a steak. (If this were real, then maybe you could defend yourself against ruffians by aiming your cell phone at them.)

Acting upon a request from a viewer, the Brianiac television show ran a test on whether a cell phone (mobile phone to them) can cook an egg. To make the test dramatic, they used 100 phones, not just one. Alas, not even 100 cell phones can appreciably warm an egg.

So, how were the popcorn videos faked so that we think the kernels are being exploded by the cell phones? A microwave beam could have heated the kernels, but it would have also injured the people we see in the videos. An overhead, powerful infrared lamp could have heated the kernels, but it would have probably melted the cell phones.

It turns out that the videos were part of an advertising campaign. As the people near the phones pretended to see the kernels exploding, laughing and exclaiming in amazement, someone dropped previously exploded popcorn down onto the table and into the camera’s view. Later, the video was edited so that as each exploded popcorn fell into view, one of the unpopped kernels on the table disappeared via digital editing. The fact that the transition involved a very rapid explosion helped hide the editing. Here is one more video but as you watch, keep an eye on the foot in the upper right of the screen --- it disappears when the kernels pop, clear evidence that the video was faked.

So, cell phones will not cook popcorn. However, I must warn you that bananas are a whole different matter. Because every banana contains a trace amount of radioactive potassium, you should never leave popcorn kernels near two or more bananas. The isotope potassium-40 undergoes beta decay in which the nucleus emits an electron. If you unwittingly aim several bananas at kernels of popcorn, the emitted electrons can heat the kernels to the explosion point. You don’t believe me? Well, here is a YouTube video that shows the effect, and if the video is on YouTube, it has to be true, doesn’t it?

4.102 Lava lampsJearl Walkerwww.flyingcircusofphysics.comJuly 2010 In the 1960s, during the psychedelic music of Purple Haze by Jimi Hendrix and White Rabbit by The Jefferson Airplane, lava lamps enhanced the mood (and perhaps also the hallucinations) of partying students. Once plugged in, a low-wattage bulb heated the bottom surface of a container holding a mixture of oil and salt water. Soon, the oil formed blobs that gradually floated to the top of the container and then gradually sank back to the bottom. The cycle was repeated indefinitely in a mesmerizing dance. Here is a photograph of my lava lamp

and here is one of several videos available on the web, with music by Peter Gabriel

Recently Balazs Gyure (of Lorand Eotvos University in Budapest, Hungary) and Imre M. Janosi (of Lorand Eotvos University and also the University of Minnesota in Minneapolis, Minnesota) conducted what appears to be the only thorough investigation of the physics of lava lamps. A container of silicone oil and salt water (distilled water with sodium chloride) was heated along the bottom surface by flowing hot water and cooled along the top surface by flowing cold water. The oil and salt water are said to be immiscible, meaning that they do not mix.

The researchers found that to set up the lava-lamp convection of the psychedelic age, they had to careful adjust the properties of the mixture and studiously clean the container’s interior before the liquids were poured in. (Any contaminations on the container’s wall would pin the oil so that it would not move.) The volume ratio of oil to salt water was also crucial. For most ratios, the two liquids simply formed stationary arrangements, with one above the other or with both spanning the height of the container. Here are some drawings based on the photographs in the research paper.

After careful adjustment of the volume ratio, the desired cyclic circulation began. The key to the circulation lies in how the densities of the two liquids depend on temperature and the fact that at one critical temperature (the cross-over temperature), they have the same density. At lower temperatures, the density of the oil exceeds that of the salt water, and at higher temperatures, the reverse is true.

Initially the denser oil lies on the bottom. As it gradually warms and becomes less dense, its temperature exceeds the cross-over temperature and it becomes lighter than the water. Eventually it breaks free of the bottom and floats to the top of the container. There it gradually cools below the cross-over temperature, becomes denser than the water, and eventually breaks free of the top and sinks to the bottom. Then the cycle is repeated.

If you are unfamiliar with the psychedelic music of the 1960s, I recommend Time by the Chambers Brothers (the long version), I had too much to dream last night by The Electric Prunes, and anything by Jimi Hendrix, especially Purple Haze.

4.103 Pub trick – making a coin rattleJearl Walkerwww.flyingcircusofphysics.comSeptember 2010 The challenge this month is about a coin placed on the top of a beer bottle. Can you make the coin rattle without touching it or rattling the bottle or the table, and without even being close to the bottle?

Here is a hint to delay the answer --- both the answer and the physics are simple and something you heard about in high school chemistry and physics glasses when you learned about an ideal gas and the ideal gas law.

Open a cold bottle of beer and pour its contents into a drinking class. Then dip the coin into the beer. As you bring it out of the beer, spread the clinging beer over the full surface of the coin. Then place the coin over the open top of the bottle and step away. Within a minute, the coin begins to occasionally rattle up and down. You can see all this in the following video:

The air that flows into the bottle as you empty the beer is at room temperature but after the bottle is set down, the air begins to be chilled by the cold wall of the bottle. By the time you place the coin on the bottle, the air is at about its lowest temperature. For the next several minutes, the walls and trapped air begin to warm toward room temperature. This temperature increase causes an increase in the kinetic energy of the air molecules and thus also an increase in the air pressure.

If you do not wet the coin, then the coin forms an imperfect seal on the top and the increase in air pressure merely pushes some of the trapped air out through the tiny openings between the coin and the top of the bottle. However, with the coin wet, beer fills those openings and completes the seal.

Every now and then, the air pressure increases enough to lift the coin, momentarily breaking the seal and allowing some of trapped air to escape. As the pressure drops slightly, it is then not high enough to maintain the lifted edge of the coin, and so the coin drops back down on the top of the bottle, striking the glass with a noticeable sound --- part of the rattle. This process can be repeated many times (and in short bursts) until the air pressure is no longer able to lift the coin and the internal temperature is approximately room temperature.

I used an empty beer bottle, chilled it for 30 minutes in a freezer, and then placed a water-coated United States quarter (a coin of value 0.25 US$) over the top. The rattle of the coin began almost immediately and reoccurred frequently for about 10 minutes.x

4.104 Pub trick --- lifting a shot glass with your palmJearl Walkerwww.flyingcircusofphysics.comSep 2011 Half fill a shot glass with strong whiskey (or some other strong alcoholic fluid). The challenge is to lift the shot glass by using only your palm, not the rest of your hand. Thus, you cannot simply pick up the glass with your fingers. Here is a hint: the alcohol vapor is flammable.

The flame produces hot gas and heats the air inside the shot glass. When the rim is covered by the palm, the flame quickly goes out, allowing the gas that is then trapped inside the glass to cool. That decrease in temperature means that the average speed of the gas molecules (moving randomly with thermal energy) decreases.

The pressure of the trapped gas on the interior surfaces (whiskey, glass wall, and your palm) is due to the random collisions of the gas molecules with the surfaces. Because the average speed of the molecules decreases, so does that pressure. The internal pressure is then less than the external atmospheric air pressure, and the pressure difference forces the glass onto the palm, causing the palm to form a tight, gas-proof seal around the rim of the glass. The rim is pressed onto the palm (or, if you like, the palm is sucked down onto the rim) so much that the glass and the whiskey can be moved and rotated.x

4.105 Pub trick --- burnt wood stickJearl Walkerwww.flyingcircusofphysics.comFebruary 2012 Push the wood end of wooden match into an empty cardboard box (such as a matchbox) so that the match stands vertically. Place a coin on the box and then place a second match so that its wood end is on the coin and its head leans against the head of the first match. The challenge here is to remove the coin without touching the matches (or blowing the leaning coin off the box).

Here is the answer. With a lit third match, set the leaning match on fire near its top end.

It quickly lights the vertical match and the two heads are fused together by the heat. As the leaning match burns, its lower end lifts upward, off the coin. Then, of course, you can remove the coin without touching the matches.

So much for the trick. The real question here is: Why does the burning match curve upward and off the coin?

The answer has to do with the temperature distribution in the section of wood that is burning or has just finished burning. The burning section is probably black and not especially hot. The section that has just finished burning is red on top and black on bottom, revealing that the top surface is especially hot. Both top and bottom surfaces are cooling, but the top surface is cooling from a higher initial temperature (it was hotter in the flames because convection causes air to flow upward along the side of the stick). During the cooling, both top and bottom surfaces contract (as most objects do when cooled). However, because the top surface cools from a higher initial temperature, it contracts more than the bottom surface. This unequal contraction causes the stick to curve, becoming concave upward.

4.106 Solar prominences and filamentsJearl Walkerwww.flyingcircusofphysics.comMarch 2012 The turbulence associated with the rising hot gases in the outer atmosphere of the Sun can cause magnetic field lines to extend outward from the Sun, producing magnificent loops of hot (high speed) ions. If such extension is seen against the dark of space, it is called a prominence and we see it because we intercept some of the light the ions emit. When seen against the bright surface of the Sun, it is called a filament. It is then visible because it absorbs some of the light from the surface that otherwise would reach us.

David Saroff, a graduate student at Rochester Institute of Technology in Rochester, New York, has very kindly collected some beautiful examples of prominences, to be shared with the FCP audience.

notebooks of ~100 interesting events (this is a zip file; the others are mp4 files):

4.107 Setting fire to an orangeJearl Walkerwww.flyingcircusofphysics.comJuly 2012This video, which has had over four million hits, severely tests the gullibility of its viewers. In essence, it asks us to believe that water will burn. A man cuts out a cavity on an orange, places a small stone in the cavity, places one end of a stick on the stone, and then twists the stick against the stone a few times. Suddenly the orange begins to burn. Here is an image showing the stick inserted into the orange:

What is wrong with the physics and chemistry here? I’ll start the list but you might be able to add to it.

1. Although you can rub stick against stick to start a campfire (boy scouts and girl scouts learn to do this), the process is actually difficult because the frictional heating is small. Also, you need kindling (very thin, combustible) materials nearby. The material must be thin so that it can be easily heated to its ignition point. A thicker material would need a much greater supply of thermal energy, an impossible requirement of stick-on-stick rubbing. In the video, there is no kindling and the stick is turned against the stone only a few times. Such frictional heating would be imperceptible to the touch and certainly would not set kindling on fire.

2. The material surrounding the stone is primarily water. If water could burn, why would we use it to put out a fire?

So, how is the person able to demonstrate a fire in the cavity in the orange? One clue is the blue color of the flame, which is characteristic of burning lighter fluid as used in cigarette lighters. Rerun the video and watch how the camera moves off the orange early on and then again just before the flame is seen. The first time is to convince you that the camera operator wants to focus on the man’s face, so that the second time he focuses there, you don’t think about it. But during that second time when we cannot see the orange, someone has poured lighter fluid into the cavity and set the fluid on fire. When the camera comes backs to the orange, we see the flame.

Some of the four million viewers who watched this video realized it is obvious nonsense, but many were stunned and impressed. If you challenged them, a probable response would be, “Well, anything is possible.” Actually, no, not everything is possible because events and phenomena follow certain rules, scientific rules. And from those rules and just everyday experience we know that water (not even water with a bit of orange juice) can burn.

If you really want a burning orange, you can make an orange into a candle. As shown in the following videos, first cut the orange along the “equator” with the stem at one “pole” of the two hemispheres. Cut through just the skin, all the way around. Then use a finger to separate the skin from the pulp, all the way around, until the pulp pops away from the skin.

One of the now empty hemispheres contains the internal part of the stem. That stem will be the wick for your candle. Place that hemisphere on a table and pour a small amount of cooking oil around and on the stem and then give the oil time to be pulled by surface tension up into the stem. After the stem is soaked, light it (this may take some patience). The flame vaporizes the oil at the top of the stem, and the oil vapor then burns. Surface tension continuously pulls oil up through the stem from the surrounding pool to replace the vaporizing oil.

Next cut venting holes in the other hemisphere so that you can place that hemisphere over the first one, to complete the sphere. You then have a lovely decoration—a lit-up orange.

As with a candle, most of the light is due to the formation of solid carbon particles which are heated as they rise into the luminous zone. There they are so hot that they are incandescent, emitting in the long-wavelength end of the visible spectrum. We see the yellow color characteristic of candle flames. Around the base of the flame, blue light is emitted by carbon and hydrocarbon molecules. Chemical reactions produce these molecules in their excited states, and they emit blue light when they de-excite to their lowest energy levels.

4.108 Pub trick --- wine moving up into an inverted glassJearl Walkerwww.flyingcircusofphysics.comApril 2013 Putting wine into an upright glass is easy enough. The challenge here is to put wine into an inverted glass. Obviously you cannot pour wine upward. So, we need a trick, something that will push the wine up into the inverted glass. Here is a video that demonstrates the trick, but can you explain why the wine flows into the inverted glass?

The procedure in the video is this: A cherry is placed in a dish partially filled with wine and then several matches are inserted upright in it. (A lemon slice would serve just as well.) After the matches are lit, the inverted glass is lowered over the burning matches so that it is supported by the dish. As the matches burn out, wine flows under the rim (it is not tightly set against the dish) and up into the glass.

The question is why? And the answer is that the internal air pressure decreases, and then the greater external air pressure pushes wine into the glass. Ah, but why does the internal air pressure decrease?

Here is a very common, but wrong, reason. As the matches burn, they consume the oxygen in the air and thus remove it. With fewer gas molecules in the internal air, the air pressure must decrease.

Well, yes, the oxygen is consumed but the combination of that oxygen with the hydrocarbons from the wax vapor produces carbon dioxide gas. The result is a small decrease in gas volume, not enough to account for the inflow of wine.

Here is the correct answer: The flames produce hot gass. If you lower the glass over the flame quickly, some of the hot gas will bubble out from under the rim of the glass. Because I cannot see any bubbles in the video, I think that the gas surrounding the flames was already heated by the time the glass was lowered.

Eventually, most of the oxygen inside the glass is consumed and the flames go out. Then the internal gas cools and contracts. At the same time, some of the water vapor condenses out on the glass or the wine. (You can see the condensation on the glass in the video.) Both processes reduce the gas pressure inside the glass. The greater air pressure outside the glass then pushes wine up into the glass.

4.109 Exploding manholesJearl Walkerwww.flyingcircusofphysics.comMay 2013 In most American cities, the electric power supply runs through conduits beneath the streets. As the insulation and support materials for those electrical cables age or become compromised by salt from street de-icing, they become brittle and cracked. Electrical faults can then develop with large currents running along the sides of the cable or jumping between cables or to the surrounding vault. The heating by the currents can vaporize the organic compounds forming the cable insulation, which can then burn. If the burning is incomplete, a black smoke can escape around the manhole cover, but if the burning is rapid enough to be an explosion, the cover can be launched into the air as a high-speed projectile. Here is a video showing the smoke, with periodic explosions:

Here is a video showing an explosion that caught a worker by surprise. Warning: There is a mild curse word in the audio. (I would have used stronger language if I had been standing next to the explosion.)

In this next video I want to warn you that a person is seriously hurt by a manhole explosion. The person appears to light a match on the manhole cover. In the latter part of the video, you can see how high the manhole cover is thrown. The cover is quite heavy and so the explosive power in launching it was significant.

These manhole events, as they are called, are fairly common in some cities, such as New York City. Because the events are obviously dangerous, researchers study their physics and chemistry to find ways to decrease their occurrence or at least their severity. One suggestion is to use a manhole cover with a greater diameter. The heavier cover would then be more difficult to launch but, more to the point, it would allow a larger escape route for the volatile gas inside the manhole. So, the gas would readily pour out around the cover once the cover begins to move upward. Another suggestion is limit the flow of air through the ducts and into the manhole. The best solution is, of course, to use insulation that is long lasting, even when exposed to water, mud, and salt.

4.110 Pub trick --- picking up a shot glass with your palmJearl Walkerwww.flyingcircusofphysics.comNovember 2013 Challenge: can you pick up a shot glass (holding a shot of whiskey) by using only your palm and not your fingers? Obviously mashing your palm down onto the rim will not make the rim stick to your hand. So, you have to be cleverer than that. Here is the video solution:

You pour a shot of 100 proof alcohol into the glass (the liquid is 50% alcohol). Then you ignite the alcohol vapor rising from the liquid. Once the flame is set and the gas within the glass has warmed, press your palm down onto the rim to cut off the oxygen supply. The flame is extinguished almost immediately and the warm gas trapped inside the glass by your palm begins to cool. When the gas was being heated, it expanded, which pushed some of the gas out of the glass. Because you have sealed off the rim and disallowed gas from flowing back into the glass, the pressure of the trapped gas decreases as the gas cools.

With reduced gas pressure inside the glass, the flesh of your palm is pulled downward into the glass, which firmly seals the rim. One way to describe the effect is to say that your palm is sucked somewhat into the glass. You can then pick up the glass by merely lifting your hand.

x

4.111 Window frost flowersJearl Walkerwww.flyingcircusofphysics.comFebruary 2014 When the polar vortex recently dipped down into the American Midwest and the temperatures here in Cleveland dropped well below freezing, the intense chill of the day was partially offset by the beautiful ice formations on my windows. Here are some of my photographs:

The formations are often called frost flowers because they can resemble ferns.

In the winter, my windows have an outer glass layer (a “storm window”) to help decrease the energy loss from the warm rooms to the cold environment. When the air temperature on the storm window’s outer surface is below zero and there is sufficient humidity between the storm window and the normal window, some of that humidity condenses onto the storm window’s inner surface and freezes. Scratches, dust specks, and other nonuniform features of that inner surface determine where the ice forms because the formation requires nucleating sites. Ice crystals can more easily begin along a scratch than on a perfectly smooth plane.

However, once the ice crystals begin to form along a line, they can branch out to form the fern shape. Exactly what shapes occur depends on several factors, such the actual temperature, which depends on not only the external air temperature but also on the sunlight exposure. I usually see a variety of different shapes on each window, suggesting that the temperature varies across the window.

When I raised the normal window so that I could photograph the frost flowers, my breath added to the moisture next to storm window. When I then lowered the normal window, I saw new frost flowers beginning to form in regions of the storm window that were previously clear. I felt like an artist, although I knew that my art would never survive any warming of the glass by sunlight or increased air temperature.

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4.112 Friction weldingJearl Walkerwww.flyingcircusofphysics.comOctober 2014 When two surfaces are forced to slide against each another, that external force can result in raising the temperature of the two surfaces. You might have noticed this in common situations. For examples, a match ignites when rubbed across a rough surface, and you are burned when a rope suddenly slides through your hands.

The friction between the two sliding surfaces is due to the formation of molecular bonds between the two surfaces. As the surfaces slide past each other, those bonds are quickly established and then broken by the forces maintaining the sliding. Each bond break releases energy into the vibration of the molecules on the surface. The temperature of the surfaces is a measure of the collective vibrational energy. As bonds are broken the temperature increases.

Here are two video examples in which two metal cylinders are forced to rotate against each other as one is gradually pressed against the other. The sliding quickly heats up the two surfaces to the point where they melt and become fluid. Because the pressure of cylinder-on-cylinder is maintained, the fluid is squeezed outward where it forms a collar. When the rotation stops, the collar quickly solidifies and the two surfaces are then bonded. Thus, two surfaces can quickly be fused by this technique of friction welding. Because the surfaces do not require any special treatment or cleaning, this type of bonding can be much easier than conventional welding.

4.113 Pub trick --- using thermal stress to open a wine bottleJearl Walkerwww.flyingcircusofphysics.comFebruary 2015 How can you open a wine bottle when the cork has deteriorated and thus you cannot embed the normal corkscrew to pull out the cork? Here is a traditional way that is still used in high-class restaurants. Wipe off the neck of the bottle. Place a brush in icy water. Position the ends of a pair of tongs in a flame until the ends are very hot.

Then clamp those ends onto the neck of the bottle for about 30 seconds.

Put the tongs safely aside and then use the brush to coat the neck with cold water.

The heat from the tongs causes a rapid and uneven expansion of the glass where contact is made. The cold water causes a rapid and uneven contraction of the glass. The glass already had defects and micro-fissures. This rapid expansion and contraction widens the fissures and extends them through the glass. The neck is then weak enough that the top can be broken off. x

4.114 Tire explosionsJearl Walkerwww.flyingcircusofphysics.comFebruary 2015 The physics here is easy: If you inflate a tire too much, the highly compressed air can dramatically explode outward at the rim on which the tire is mounted, or the tire can rupture like an over-blown balloon. If the tire and rim are lying on the floor, the air exploding toward the floor will lift the tire and rim upward as though they are a rocket. In this first video, you can tell that the explosion is powerful because the tire and rim fly up to the ceiling. The worker who had been pressurizing the tire was lucky that the tire and rim did not land on him.

4.115 Coffee heated in hot sandJearl Walkerwww.flyingcircusofphysics.comJuly 2015 Turkish coffee is brewed with water, sugar, and very fine grains of coffee in a small container called an ibrik (or a cezve, as in the following video). The mixture is heated until a foam forms on the top, and then the foam is poured off into a small cup. This procedure might be repeated several times before the ibrik is empty. Here is a vendor that brews the coffee by placing the ibrik on or somewhat inside a bed of sand that lies in a large metal pot which is heated by a gas burner. The question is, why bother with the sand?

It distributes the thermal energy from the hot plate over a larger surface area. If the ibrik were placed directly on the gas burner or even just above it, the energy transfer to it would be too rapid and the coffee would overheat and foam without adequate control.

Because the ibrik has a short handle, placing the ibrik directly over the gas burner would require the coffee brewer to expose his hand continuously to the thermal radiation from the burner. With the sand, the ibrik can be put in place and then the hand withdrawn for a few seconds.